What size bearings should you use for Tech Deck wheels?

If you are talking about making them custom, you should probably buy one and make a presicion mold. If you want one just for your fingerboard, skip the standard polyeurethane wheels(they dont have them, and wont fit bearings inside them), you want expensive bearing wheels from Z shred or Jace. Either way, you'll have to buy stuff. Making them yourself is strictly not reccomended, 1: it's super hard. 2:you probably can't do it, and 3: it's easier just to shell out a buck or 2.