The're 6X10 mm . You'll need 2. My LHS had them both in flanged (big on one side), and unflanged. You need the unflanged style. (Normal looking bearing). You should oil them when installing them and on a regular basis after that. In fact, On mine, if it moves, I oil it. I got into a practice after a flying session to clean the whole machine, and doing the oiling then. As well, after every 5th or 6th session, I remove the tail assembly and grease the wire drive, as well as the tail gearcase.
Heli's are a lot more high maintanance item than a plane. Do it regularly, and you'll get good service out of it.