I wouldn't worry about the size. Use same size as you have (1/4"?) . You may be able to get an extra ball in there without the cage taking up space. Just make sure there is enough space between balls to allow free movement. Assembly can be a little tricky without the cage since races are perpendicular to ground so gravity is working against you. Use enough grease to hold balls in place in the fixed cup until you can get the spindle in there. Do same with adjutsable cup. Tighten until play is gone. To make balls last longer, store bike in dry place (get water out of frame if you ride in the rain), periodically check adjustable cup to make sure it is adjusted properly, and regrease bearings periodically (for this latter part, no need to remove fixed cup; just remove adustable cup and spindle and shoot grease into fixed side from the adjustable side).