Testosterone will do nothing to enlarge your penis if you have reached adulthood. However, it will result in stronger erections. higher sex drive, and possibly more aggressive behavior.
Testosterone administered prior to or during puberty may increase penis adult size, by starting development sooner. I have a friend who grew up outside the USA, where medical practices are a bit different. He distressed his father by exhibiting "homosexual tendencies" as a small child. So, the father found a doctor who prescribed therapy with hormones, which I assume included testosterone. He grew up to be gay anyway. He did end up with a huge dick (but is a total bottom), tiny balls, and suffers from depression, all of which are likely attributable to this "therapy."