The prostate on the average male can shrink significantly with finasteride usage. This could easily give one the illusion of a smaller penis.
Of course, low testosterone also could cause shrinkage, or anxiety and inflammation of the prostate or prostatis, or even hematuria, or worst yet prostate cancer.
That's why self-medicating yourself and/or depending on a side effects forum for medical advice could be a dangerous proposition. If the original poster truly does have a problem it would be wise to advise him to seek help. I'd suspect a GP might perform a blood test and check your hormone profile etc. And then you could possibly be referred to a urologist for a possible prostate draining etc. Who knows?
Good luck.
