This 1959 study is a case series where every subject who had penis size increase were ‘eunuchoid’ from a pre-pubescent age and did not have normal penis development because they were hypogonadal, ie, didn’t make testosterone, or made so little as to not be androgenized.
These men who had ‘infantile’ penis size essentially underwent puberty when given a DHT-derivative and their penises grew to ‘normal adult size.’ The ones who had hypogonadism arise later in life (as post-pubescent males) had no report of penis size increase.
So essentially, this set of observations goes along with the general understanding that adding androgens after puberty has little or no effect on penis growth. It can help with erection quality and frequency, so it might help prevent penile atrophy with declining androgens, but so does any kind of TRT, some of which will be converted to DHT.
So if went through puberty you won’t grow from it. If you have normal sex drive from normal test levels it won’t help you keep your size.