I`m new to forging, but stock removal was my bread and butter (Literaly how I earned a living) for years. The smaller wheel creates a deeper dish in the blade and just makes it easier to find your groove again and again. 8" seems to pretty much standard, but I've known guys to use as big as 12" and as small as 4". It`s all personal preferance.
Still, I find it easier and more relaxing to flat gind on a platen or a disk instead.