First.... DO NOT grind on metal wheels! It's just asking to shred a running belt, and trust me, it hurts.
Metal wheels are intended as only "idler" wheels to support the belt. I admit that I get a bit tense whenever I see outfits selling metal wheels as "contact" wheels..... IMO, those people obviously have no grinding experience, and don't realize/care that they are selling a good way for an unknowing knifemaker to hurt themselves.
As far as sizes, I once had everything from 1/2" through 1", and found that the only one I ever used was a 3/4". Of course the exact size(s) a person needs/uses, is dictated by what the individuals needs....but I ended up selling all of my small wheels except the 3/4"...... I keep a smooth and serrated 3/4", along with a spare of each these days.