Is there any proven way to make your dick smaller
 

Without any other side effects

Not that I know of. Not permanently. Denial does work in the short term. If you can deny yourself erections and orgasms for a week or more. Wear a chastity cage and take cold showers. Your limp dick will look tiny. It has worked for me.

Problem is, it all comes back after one good hard-on. I keep being told it shrinks permanently if you do it for long extended periods, but I think that might just be people's fantasies.

No guarantees, but any medication that can cause impotence MAY lead to shrinkage over time. If that muscle doesn't get exercised, then it's going to atrophy (shrink).

You'll have spotted the flaw though - in order to get the shrinkage, you have to put up with the impotence, which I guess is not what you're after. I don't think there's any way to get one without the other. It's a nice thought though - a micro-boner. Wouldn't that be a sight to see!

I've been functionally impotent (unable to erect enough to penetrate) for some time and rarely wake up with 'morning wood'. I'm sure there has been some slight shrinkage over time, but it's difficult to say as my weight has increased as well...