It depends what results you want, right? Draw out a 14 inch circumference on a piece of paper, cut it, and apply it to then end profile of your intended bar stock.
This way you can see what your grind will look like in profile.
Thickness and height all play into it.
I have the 14 and absolutely love it, but you wont get that short deep hollow with it. If that is what you are after, 8 or 10 would be more suitable.
A lot of guys use the wheel to do an initial grind on their stock, even if flat grinding. It's supposed to extend the life of your belts. I've never bothered with it, but you might hear some good input from others.