RE: What power for a .20 size Citabria?
For that size plane, you have many choices. From my experience, I would recommend an outrunner motor like an [link=http://www.hobby-lobby.com/brushless_axi.htm]AXI 2820/10[/link]. The simplicity of a maintenance-free design having no gearbox is a great choice for new electric enthusiasts. Even my high end geared motors always seem to develope some gearbox issues.
Use an APC 11x7 e-prop on the 3s2p [link=http://www.hobby-lobby.com/thunderpower.htm]ThunderPower 4200mAh[/link] pack and you'll be flying for 30 minutes.
Good luck!