Injections (or any of the alprostadil vasodilator drugs) tend to be dished out at the rate of once a week for having actual sex. Occasionally people have got more for the purpose of penile rehabilitation rather than having sex, but not often. I think to preserve penile structures, you should be aiming for erections more often than one a week. Reaching orgasm is not necessary for this purpose, but go for it too if you want to.
The RP will have lost you some length anyway, typically 1" - 2". This is due to shortening the urethra, because you lost the part of it inside the prostate, and to rejoin to the bladder, some is pulled in and the bladder neck is pulled down to meet.
If you don't have regular erections (and this applies both after RP, and also in the case of loss of libido for men on hormone therapy), the corpus cavernosum (erectile parts of the penis) start forming fibrous tissue, which reduces elasticity. This causes less expansion both in length and girth, and can cause less hardness too. The flaccid penis is ischemic (short of blood flow - only 30% of similar body tissues) and relies on regular erections to get a periodic good blood supply to the tissues.
Of course, it's up to you how much effort you put in, and how important it is to you to try and prevent your penis getting shorter. Once fibrous tissue forms in the corpus cavernosum, that's not reversible, although you might get a reversal of some very recent shortening when you start using a pump.
Something else that may help (but is difficult to get in many areas) is a daily low dose (5mg) tadalafil prescription.
What I would suggest is that you get a referral to an ED clinic, and they can go through the options, and what's available in your area. There may be some special considerations relating to being on an anti-coagulant too.