Other then allowing for the different radius each wheel provides, the reason I went with the sizes I did was because it "fit" my knifemaking style....by that I mean that whenever I cut a radius at the bottom of a ricasso (to allow for "Choking up" on a knife) the 1 1/2" radius is what I like. The reason I went with the 3" at the bottom is because thats the radius I like on the "heel" at the bottom of most hunter sized knives.
Although I did design/build a "special" platen arm, its not really necessary. You can use different sized contact wheels at the top and bottom of the stock KMG platen..... you just have to rotate the platen a bit if you want the belt to run vertical across the platen. Possibly the only thing you MIGHT want to do, based on how small you go with contact wheel size, is to "trim" around that particular mounting hole so you have better/complete access to that contact wheel.
Just a quick note on contact wheels. I've come to like the Urethane wheels much better then rubber ones...... they last MUCH longer, and just seem to be overall "tougher" then rubber. The only drawback with them is that SunRay Inc (where I purchase the majority of my contact wheels) doesn't keep them in stock, but rather makes them to order. That means you have to wait, generally 4-6 weeks for your contact wheels to ship. ALTHOUGH.... I think I do remember seeing urethane wheels on USA Knifemaker.... so you could check there too.
