What causes penis to change size after prostate surgery?

This is a rather interesting topic because the word on the street varies widely from the medical consensus. Here is a stab at why that is the case:
1) The penis (actually the nerves connected to the penis) is traumatized by the surgery. Even if everything is intact, it takes a long time (multiple months, sometimes years) for the nerves to return to more normal function.
2) Since some of the nerves are right on the surface of the prostate, even the best "nerve-sparing" surgery will likely damage some of the nerves. By damage here, I don't just mean "trauma," but rather that the nerves may actually get cut, broken, or whatever, and no longer send signals (thinking of them like electric wires.)
3) When the prostate is removed, the connection between the bladder and the penis is cut and then reconnected. The difference in "length" is miniscule, so while this is true, it is not what we're all noticing.

So, my penis looked like a scared rabbit (metaphorically speaking) after the surgery, but now as I approach two years since surgery it looks a lot more like I remember it a lot more often. For a while, I had a hard time getting it to relax enough and have enough blood flow to pee standing up, even though I follow my physical therapist's advice to drink large quantities of water daily.

My friend who had first prostate cancer and now bladder cancer (over many years) says his situation is much worse.

So that's the story. It may not be physically shorter and smaller, but shorter is a reflection of the nerve status and smaller is a reflection of the blood flow, and it all interacts. Drugs like cialis and viagra work by increasing blood flow based on a chemical reaction involved, and they are often prescribed after prostatectomy to help with the recovery process. Less commonly, vacuum devices or a physical tensioning device (RestoreX) may be used to help get blood back into the penis during the months long recovery period after prostatectomy. This is because if the "smooth muscles" in the penis atrophy for too long, it can be difficult for them to recover, so all three of these strategies are designed to keep those muscles from wasting while the nerves recover.

My experience was obvious penis shortening after arriving home immediately after my LRP at age 53. As if it was pulled into my body. Never regained size regardless of the treatments available. The two urologists that were involved with my decision to proceed with the LRP did not communicate this possibility. It’s my opinion that had I known I would have considered other options.

My urologist explained that the bladder drops during surgery for sewing the urethra back together. This makes sense to me (retired doc) as the urethra exiting the pelvis is firmly attached to the soft tissues of the perineum. I have noticed penile shortening mainly associated with ADT/Lupron therapy. After surgery, if the penis doesn't have daily/frequent erections the muscle atrophy and scars, shortening the penis. The effects of the Lupron is more associated with loss of testosterone.
That's my understanding.

I beg to differ on the miniscule aspect of a prostate removal's affect. If a prostate is approximately the size of a walnut, and your urethra goes through the prostate, then you lose that much urethra after a prostatectomy. The reconnection doesn't add any artificial urethra where the prostate used to be. That walnut sized difference doesn't get made up for. My surgeon said the hope is that the bladder will drop towards the penis and not that the penis will draw up towards the bladder after the reconnection, but there are no guarantees. While I am sure you are correct about the atrophy and the nerve damage, I think we have to attribute more than a miniscule amount of penis shortening to the loss of the prostate.

I had read that when the prostate is removed, the urethra becomes shorter. One of two things happens: the bladder is pulled down or the penis is pulled in. I guess I’d rather have my bladder stay where it is. I had RP in November and my penis has shrunk. I have to lean over the toilet so I don’t pee on my clothes. Holding it becomes more of a science. It’s functioning fine(knock on wood-no pun intended). Hope this helps.