The size of ESC required is determined by the current your motor will draw, with your prop and gearing, from your battery. You, first, need to measure or otherwise calculate the draw your set-up will achieve, and then select an ESC with a current rating above that. Most ESC's have two ratings given - one for continuous, the other for very short term bursts. ALWAYS err on the side of caution - there is no harm in having a higher rated ESC than you need (the only downside will be that the higher rated the ESC, generally, the heavier it will be).

Brushed motors need brushed ESC's, brushless motors need special, generally more expensive, brushless ESC's. If you use Lipoly batteries you should get an ESC which has built-in or programmable LVC (low-voltage cut-off) to save your packs from over-discharge. Castle Creations with their Pixie (brushed) and Phoenix (brushless) are among the leaders here, though Jeti and others also make suitable ESC's.

Hope this helps.

Cheers, Phil