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....
Type/size of bearings are hub make/model dependent. For cartridge bearings, either measure OD/ ID/ width, or read the designation(usually 6000-something) before you go shopping. For cup&cone loose ball bearings either measure ( you’ll need a vernier caliper for that, or look up the spec for the hub. For caged ball bearings, I’ve gotten away with size and number of balls.