[jannet.duff]

If the penis does indeed end up shrinking from lack of use.. when you do use it occasionally and get hard.. will it then start to grow again or is it a permanent shrinking effect?

Its more of the skin that shrinks than the penis. During normal sleep cycles a male can get hard several times a night ( ever woke up horny with a hard on for no reason ? ) With lower than normal testosterone, and higher than normal Estrogen, those nightly cycles disappear, on top of that if your libido dies off then your "manual excersise" will be greatly diminished too.
After a few months everything will shrink down. Now the problem arrises when you havent used him in a few months and out of the blue you get an excited thought... It'll hurt like  fuxxxng hell.. Your actual penis will try to grow bigger than the skin surrounding it can expand.

The general rule of thumb is use it or loose it !!