Could masturbating often cause a decrease in size?

Highly, highly doubt it will stunt your growth, per se. It's very possible though, that you can tire or overwork your dick if youre masturbating multiple times a day, every day of the week. At least this is what I believe to be the truth -- I think I've probably experienced it at times in my life, and I've heard this from others too.
 
I actually tend to think the opposite.

If you think about it if your cock is under pressure (full of blood) for longer while you are still growing it would makes sense for it to try and make those cells bigger to releive the pressure I think wack away my man and do it often.