70% of the wheels sold for a grinder are 8", after that it goes to 10", then 14" and then 12". I think this is because that is the way it was 30 years ago and things tend to stay the same. Personally, starting out now and only had one wheel to choose from, I would pick a 10". When you get above a 10" wheel, they start to get fussy with concentricity, run out, the vibrations and the increased mass they have. All of that can be solved with tighter tolerances and more money when building the wheel. I have a 6, 10, 12 and 14" to chose from. I used to grab the 8" first but now tend to pickup the 10" first. If I am grinding thin stock, I grab the 14" to get the grind line higher on the blade.
most guys will find a smaller wheel easier as the groove tends to be deeper and once the wheel is settled in the groove it tends to stay there.