Shrinkage is normal. But the good news is that normal length usually returns with erections.
Surgery shortens the urethra by about an inch, and because the penis is firmly attached, the shortening draws it too back into the body.
However return of erections, however you do it, draws the penis out, and the bladder tends to drop down into the hole that was the prostate and the missing section of urethra. I won't say this always happens, but it did in my case.
If you want to speed things up, get a cheapie pump (VED) and start pumping!
Age 63. Other than cancer, in good health; BMI 20
Pre-op: No symptoms; PSA 5.7; Gleason 4+5=9; cancer in 4 of 12 cores
7 March 2008, RRP, non nerve sparing
Two nights in hospital; catheter and staples out after 7 days
Continent, no pads needed from the get-go
Post Op: Stage pT2 M- N-; clear margins and lymph nodes; Gleason 4+4=8; prostate weight: 37gm
6-week and 7-month PSAs: 0
Bimix injections working well