I’m a top so I can’t speak entirely from the bottom perspective but I can go by what they’ve told me.
I’ve made 5 guys cum hands-free so far. And one of those guys I’ve made do it a few times. Whether it’s my length or girth is hard to answer and I think it depends on the guy if it factors much altogether.
First guy I made do it, I know it was my length because I felt self-conscious about my size and feared if I put my whole cock in him that it might be painful for him. The position the entire time was missionary with his legs wrapped around my lower back. After fucking him a while with maybe 3/4 of my dick in, which he was clearly enjoying, he practically begged me to insert my entire length so I obliged. His moans and groans grew exponentially after that, often letting out a “it’s so big” or “omg it feels so good”. When I put the full size in, he said he could feel my tip poking his stomach from inside, so I guess I was quite deep and he really enjoyed that I poked so far inside him. After full length he did last much longer and exploded on himself with impressive volume, he spit a few streams up to his neck. Naturally I could not last much longer after getting that reaction haha.
Second guy was much like the first, as in same position and same reactions really. But he seemed to insinuate that my girth was the big factor and the rhythm/position.
Third guy actually rode me, took him a while to fit the full salami but he rode vigorously for 5 minutes before coating me with quite a volcano of cum haha. It was hot but then awkward because I’m usually nowhere near ready to blast off after just 5 minutes. So 15 more minutes ensued of my pounding his tight wrought hole before I finally reached my own volcano inside him.
4th guy had wanted me to fuck him for quite some time since he’d heard about my ability to poke the stomach with my length haha. I finally relented and said okay, I’ll fuck you and let you experience what you’re asking for. First times I tried my girth was too thick to fit, so he spent about 2ish months with a dildo that was a bit under my girth. It worked though, because after that prep, even though it was still a very snug and tight fit inside him, I was able to slowly get every single inch in. I used my tried and true missionary position with legs wrapped around my lower back for most of the fucking, we also tried riding, doggy, and on the side from the back. He actually was ready to cum early on since my length and girth were just fully filling him up and hitting places inside him that had rarely, barely, or straight up never been stimulated before. But he endured as best he could to last as long as he could. We ended up having a good 30 minutes before he just could not keep his orgasm back any longer and let loose one of the hugest cumshots I’d ever seen fly out of someone during anal. Seeing that, I busted a megaload deep in him which he said made him feel stuffed to capacity as if he’d just eaten a meal, my big dick and huge load were just taking up so much space inside him I guess. He also had to rest for a few minutes after since he said his insides ached, but in a good way, from getting such a tight deep dicking haha. But I think this scenario, the hands-free orgasm was due to all the factors: length, girth, position, pace/rhythm.
The 5th guy is my bf. I’ve made him do it a few times. It seems to be a combination of girth and pace/rhythm that seems to do it for him, though he’s said the length really helps too.