Wheel collars depend on the gauge of wire that you're using for landing gear. 1/8" wire, you can get by with 3/32" wheel collars (although they'll be a little tight, and if you have a burr on the end of the wire, it'll need to be filed/sanded to get the wheel collar on). My rule of thumb is to buy 1/16" up from the size of wire you're purchasing. That gives you a little wiggle room and you can tighten down the lock nut so it'll bite nicely. Problem with that is that my hobby store sells out of the wheel collars for a specific wire size fairly quickly, so I'm always left testing and retesting, before I finally get the correct size.
Bullet connectors for motors - those are pretty standard, either 2.5 mm or 3.5 mm - the 2.5 mm are for your tiny planes, like say, a Tiny Trainer, or Mini Corsair or something similar; the 3.5 mm are for your larger motors, like those you would find on say, the Bushwacker, or Sea Duck.
