If it's that small, you'll probably want a small gearbox setup like this:

[link=http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXHUV9&P=ML]Click me![/link]

That will permit you to utilize a small 400 sized motor, be it brushed or brushless.

The biggest problem is getting the right pinion to match your motor's shaft size.

Note they suggest swinging an 11x4.7 prop with an 8.4v battery pack.

With a small brushless motor you could swing the same or larger using an 11.1v LiPo pack while still keeping the amp draw low.