I found out in my early 30s. The first couple of girls I was sexually active with in early high school either said I was huge or had a wide-eyed intro to seeing it, but as we were all young I figured they were just inexperienced and any dick would seem huge to a high school girl about to embark in sex.
Then I started dating a girl in junior year that I ended up being with until we divorced in our early 30s. She was a virgin when we started dating and was tiny and athletic with a very tight vagina, so when we had issues during enthusiastic sex, though I was turned on by feeling big for her, I figured it was because she was so small. She never said anything about me being big, even though she could give me oral sex with both hand and her mouth working my dick.
We had no idea that was unusual. Internet was just coming on when we were in college, and even then it was a while before regular (and reasonable speed) internet was available at home on a student's budget, so the only porn references I had were magazines and the occasional VHS. And she had no interest in it so she never saw any. I thought most actors were average size, as I was as big or bigger than many, maybe even most. I thought the ones I now know as ridiculously large were why porn actors had the reputation of being guys with big dicks. And because porn was hard to come by I didn't see a ton of the giants.
When I got divorced in about 2003 I started making up for lost time and started internet dating, (and dating and dating...) and by this point age-appropriate women were much sexually experienced, and becasue of our age, many were recently divorced. All of the sudden I start hearing over and over how huge they thought I was, and I had several women become regular sex partners because they were turned on by it. I had all the comments that guys dream about, like how they loved how much I filled them up, how much bigger I was than the previous guys, how they loved the pleasure/discomfort and how turned on they were by my big dick.
I even had one regular partner, who I dated off and on (with lots of drama) for almost a year and a half tell me she kept coming back because of my big dick and the great sex. She once told me while we were fucking that sometimes my big dick scared her, and she couldn't believe how much that turned her on.
As we all know, these days people are confused by porn as to what a really big dick is compared to what the average out there seems to be. I'm a solid 7.75-8 x 6-6.25, depending on the day. Perhaps typical for around here, but really big compared to the average.
Now not to say I'm enormous, especially not compared to some guys here or out there on the internet. I dated this really hot former fitness competitor who had some trouble accommodating me (according to her deep penetration was painful because her uterus was retroverted which led to her cervix position being in the line of fire) who told me she loved my cock because it was "nice and thick, but at least I wasn't huge, especially not too long." Obviously she had been with some really big guys. By that time I was pretty confident about having a big dick, so it was nice to hear I was pleasing for her as long as I didnt go too deep in some positions.
I also had a FWB who loved my cock but told me she had been with a guy who was a legitimate 9". And I believed her because she is a hospital nurse who sees a lot of dicks and is the type to measure in a way that is reasonable. Plus she thought I was big at around 8" and she was confident that the 9 incher was bigger.
So while I wasn't hung up about it, the feedback felt real after it happened pretty regularly with new partners, and although I never made it a big deal in my mind, it didn't take long for me to feel pretty confident about my dick, and during my run as a single guy I had a run of about 4 or 5 years of lots of great sex with some amazing women where they often brought it up.
Some fun stories:
There were several women in their late 30's or early 40s that used me to get their groove back. A couple of them were completely overt about wanting to fuck me because they loved my big dick and had no interest in a serious relationship. Two that come to mind were just divorced and wanting to live out fantasies about sex with a hung guy. The first time I had sex with one of them she moaned and fidgeted while we were fucking and said over and over that I was too big. It was uncomfortable. Her ex-husband was so much smaller. Then she came and came, all while telling me I was too big. After we were done and were saying goodnight she asked me when I was available to fuck her some more.
Another seemed to be trying to live out a suppressed fantasy about being a pornstar. I had sent her some pics in my underwear while we were flirt texting back and forth and she couldn't wait to get back to my place from the bar where we met the first time. She kept telling me she couldn't wait to fuck my huge cock and once we got back to my pace and got started it was like we were on set. She was going on and on about my huge, thick cock and gave the sloppiest, noisiest blowjob I've ever gotten. We fucked and fucked, going on all night, and she wouldn't stop screaming about my huge cock making her come. For a brief time we met up a bunch and afterward each night together she would send me emails and leave VM's afterwards talking about how sore she was from my big cock, how she was telling all her friends about this stud with a huge dick that she was fucking and she couldn't wait for me to fuck her sore again.
I met a different sexy, recently divorced lady for a first date at a bar and she was already pretty tipsy when I got there. We hit it off and after a couple of drinks were dancing and getting hotter and hotter. Without asking, while we were grinding on the dance floor, she grabbed my cock and she gasped and with her wide eyes she said the magic words: "Oh my god! You are fucking huge!" It went on from there until the bar closed where she couldnt keep her hands off of me and we were going at it pretty hard in the club. Unfortunately (or maybe fortunately) she kept drinking and got so drunk that the night made a wrong turn and I ended up having her stay at my place (because I was a short cab ride away her place was way out in the burbs) but I staying hands off because she was in no shape to consent. She got so sloppy and a bit crazy that night I decided to politely beg off a second date so we never actually had sex.
I also had a short relationship with a super hot lady in her late 30's. She was a high end corporate professional who showed up on our second date dressed like a hooker and when I took her back to my place she kind of gasped when she saw my cock and said, "Oh, I knew you were special!" Man, she was amazing. Turns out she was a secret swinger and wanted to bring me to some parties. She thought my cock would be a lot of fun to share. She told me "You'll be a big hit. The women will all want to fuck your huge dick."
The wonderful twist is once my wild oats were good and sown, I met an amazing woman who I fell head over heels in love with. By some random stroke of fate she was very inexperienced, and though drop-dead gorgeous (legitimately 6 feet tall and built like an athletic runway model) was a virgin when we met. She was just conservative about sex and relationships growing up, and in school was a two sport varsity college athlete that went on to get a science PhD, so she was so driven that she only ever went on a few dates and never really had a boyfriend after high school. She decided once she finished her academic career that she wanted to find the right guy, and that's when we met. I was pretty open about my past and she didn't mind at all that I was experienced.
And although sometimes, even to this day, during sex a deep thrust has her wince when the angle is wrong (as a six foot tall, slender woman she happens to have a matching long, slender vagina--perfect!), she has no idea that I've been considered big, or even huge, by many, if not most of the women I've been with. So once again I'm in a marriage where my cock is appreciated because it pleases her but she doesn't have any idea that compared to average, I have a big dick.
And that's ok with me