A few weeks back I was greasing a hub at the bike co-op and when I went to insert the ball bearings, one of the people there gave me a sealed unit that had the bearings in it. Obviously very easy to work with and now I'd like to replace my loose bearings as they wear out with sealed units.

My question is - how do you know what size to buy? I asked at two bike shops and the employees didn't know what size either. Are all 27 inch then same or all 700c the same or are there separate sizes for manufacturers etc....