There are varying strengths that docs prescribe, depending on their expectations of your need for help. Therefore, it is difficult to identify an amount to inject. After SRT, I had to ask for increasing strengths and inject larger amounts until I decided that it was better to take a different approach and went for an implant.
As long as you inject into the corpus cavernosum cavities, which span from the base to far up the shaft, the amount should not make a difference, since the medicine passes through the cavities pretty readily. Ditto for the size - it should more be dependent on what you will require given your condition, rather than the state of things. That said, if you are partially where you want to be without supplementation, I would be inclined to inject less than your normal dose.
Gleason,3+4;PSA 9.8,Nerve-sparing RRP,03/2008(Age 48 then),pT2c, 60g, neg margins; perineural & lymphatic invasion;3 lymph nodes removed,clear; seminal vesicle invasion:absent;Gleason 4 was 5-10%; PSA <0.1 until Oct 09:0.1; retest <0.1; scans clear;monthly results from Jan 2010:0.2,.2,.17,.17,.24,.31,.29,.41, IGRT SRT started 8/4/2010, PSA@5 weeks in: .17,after:.12,.10,.06, .05 since