When growing up (not a pun)/developing I was very worried about my shape/variation in girth/circumference as it was so different from the rest of the boys, in my all boy's school, when we played those 'silly' comparison games.
At rest I am a fairly constant circumference, except for the head which is bigger than the rest of the shaft, with a long foreskin which completely covers the head.
As I become fully aroused the circumferences change - around the rim of the now completely uncovered head & shaft 5.75 ", behind the head 5.5"; mid shaft 6"; around base 6.5" ..... this causes a 'curved shape' to the underside whilst being virtually straight on the top.
All my partners have had some previous experience - some have initially been able to 'take' and 'enjoy' me to mid-shaft, in most positions; some have been able to 'take' and 'enjoy' my full length/circumference from the start in all positions Those who initially found anything more than mid-shaft a 'tight fit' very quickly adapted to and 'enjoyed' my full shaft, taking a little longer to 'enjoy' the normally tighter fitting doggy position.
I have one partner, a FWBs, who had four children over a five year period, who gets the most enjoyable sensations from full, 'balls deep' penetration, in the tighter fitting positions.
A partner, a gynecology nurse, described my erect penis as "a natural dilator".
It shouldn't need saying, although from some post it might, but the surest way for mutually enjoyable sex, with any sized cock, be it circumference/girth, or & length, is to be guided by your partners reactions to those things.