Hollow grinding wheel size depends on what you want to do... In other words, there are several factors that come into play when determining which wheel to choose. Stock thickness, grind height, and edge thickness. If you do not calculate right you will have a blow out into the primary grind.
I agree that larger is better because it gives you more versatility. Why? Well, on the extreme side of things if you had a 1.5" wide, 1/8" thick blade you wanted to do a full height hollow grind on, if you tried to use a 6" wheel you would end up grinding through (i.e. hollowing it so much) mid way up the grind. So you can see that you have to determine the right wheel size for the grind and blades you are wanting to do. This is why I say larger is better, because while it may not be as deep of a hollow grind, it will still be hollow you could just go a little thinner on the edge. Of course if you can afford both get them! hehe.
I have an 8", 10", and 14" wheel and would love to get
Nathan's 36" or 48" hollow grinding platen, but that's down the road =)
Here is a hollow grinding calculator (didn't see anyone else post it in this thread yet). It will help you determine which wheel would be right for you... If I am not mistaken, the "edge scribe" is the thickness of the shoulders of your edge prior to ht.