Yes I did. I was bit jealous how big the penis of my 3 years older brother was, compared to mine. He also recognized that something is wrong with me, because my puberty didn't start well. He found out while he wanted to show me how to wank, but didn't work when I was 13/14.
He luckily motivated me going to the doc, and he sent me back to my pediatrician who gave me hormone injections over years. During that time my brother and I compared each other penises and other changes of puberty. He also wanted me to have my first ejaculation, and he tried different ways to bring me to the climax, but it didn't work until I was 17. On the other hand my brother did some kind of experiments on me, if you want, because of his bisexual orientation, I guess. You can say, it was win win for both of us.
I compared with some friends, too. When I was 7, I was into medical games with a 9 yo boy from football club. We checked our bodies like our pediatrician did during our examinations. We also compared our penises and scrotum. We took our temperatures rectally and we gave us some fake suppositories like butter or Kinder Schoko Bons and so on.
I also compared with some classmates in the locker room after sports. When I was 11 or 12 there was an upcoming discussion about circumcision. One of my mates said “Let's ask our Turkish shorty” and looked at me. First I didn't know, should I laugh or should I be shocked, but after a few seconds I only said “OK, have a look ” and I lowered my pants to show them my circumcised penis. First there was yelling from the others, because they obviously didn't reckon on my audacity. But quickly they came nearer and were really interested how it looks and how it feels. Unfortunately I couldn't report any experiences of healing after getting circumcision, because it was done very early in my life.
And when I was 14 I could compare my penis with another friend of mine who was 17. He always said it looks good the way I look and he didn't mind that my puberty didn't start so far. With him I made my first experiences in anal sex, I described it in other threads.
So in conclusion comparing my body and especially my penis with others helped me understanding my body better helped me getting aware of my retarded puberty and gave me more self-confidence.