Ball bearings replacement for Windsor 4500 After riding my Windsor 4500 for about 2 years, I got some advice to replace the ball bearings for both tires before going on a long bike trip. I plan on replacing these myself. Could someone tell me what size bearings would be appropriate for this task?

Thanks.
You should replace them with the exact same size that are already in there.  If you don't know what size, take it to the local bike shop and they should be able to tell you the size you need and order them for you.  It's likely they are sealed cartridge bearings in which case you can't replace individual ball bearings, and you have to buy new cartridges. Although I wonder why one would have to replace them after only 2 years of use. Unless you've ridden the bike a lot more than the average person would ride it, they should last much more than 2 years, especially if they are sealed cartridge bearings.