Okay, with the help of Google calculator to do metric conversions, and xkcd for some rough info on the volume of human-shaped figures, I estimate that if Stay-Puft were of ordinary human build, he’d be made up of approx 500 kiloliters of marshmallow. Guess-timating that a mini marshmallow is about 2 milliliters, that means you could make 250 million mini marshmallows out of human-build Stay-Puft.
I’d bump that up to something like 400 million for the Stay-Puft man in the movie, since he’s so fat and puffy. 