Do you really need a cushioned wheel face on the surface grinder? Seems like unless you have a splice bump you should be ok with a wheel made of MDF or cardboard. Maybe even a standard grinding wheel? I'm not trying to suggest anyone do unsafe stuff with their machines, just curious about what could work. From my not too extensive experience with the surface grinder it seems like a rubber wheel would reduce accuracy and consistency.
You want a rubber wheel, but you do what it to be harder than a standard contact wheel (about 90 durometer as opposed to 60/70).
A full hard wheel of wood or metal would probably introduce quite a bit of chatter and I imagine it would wear more quickly/unevenly.
Now, to answer OP's question, I decided to go with a 1" wheel, but before I realized that 1 and 1/8 might be better for "dressing" the belt and keeping it flat. That said, I really haven't noticed any issues so far, and have good luck just tracking the belt edge to edge. Why did I choose 1" instead of 2" (or rather, 2 and 1/8")?
For one, it's cheaper, and second, you get twice as many belts!
Now, I'm not sure that you really get twice as much grinding, as I really don't think that's how belts work, but I also figured that I was going from a 1/2" to 3/4" wide stone wheel, why not keep a similar dimension on the rubber wheel? I normally surface grind with the belts just like I would with a stone anyhow, I.E., left to right passes going front to back, rather than just making a "full width" pass on the wheel, if that makes sense.
As for splitting belts, USAKnifeMaker sells a belt splitter that makes fast and precise work of it (actually, I just checked the site, and big surprise, it's Out of Stock, like about 90% of everything else I've searched for recently...), though I just made my own, as shown in the vid below:
[video=youtube;SL8wUD_nAWA]https://www.youtube.com/watch?v=SL8wUD_nAWA[/video]
