Again I think this will depend on the individual. I have asked my uro what fills up the space vacated by the prostate and he said the bladder. If this happens, then there is no reason why the penis would appear shorter -- the length of urethra outside the body would be the same as before. This has been my experience -- no change in length flaccid or erect. Although while the catheter was in place, I looked very withdrawn.
However, if something else occupies the space -- maybe fatty tissue, then you could expect to see some shortening -- less urethra outside the body.
Just a theory from a lightweight who have doesn't fatty tissue looking for someplace to go