I bought a Jet Pilot PFD a few years back for wake boarding. Started wearing it when I was learning to kite after a couple of long sessions of dropping the kite over and over and spending most of my time bobbing around.
I wore a waste harness with it with out a problem, it actually stops the harness from riding up. You just got to make sure the vest is tight on other wise the harnes can push it up to your face.

Not good.
Best method I found is to buy one that fits and drink beer every after noon for about twelve months, by this stage it should be fitting very snugly.