20A to 25A or there abouts =) It's better to go a little heavier than to go too light. The stock motor can draw over 20A in a full stall, so if the ESC doesn't have current limiting, then you are going to want to figure out the max draw the motor would have in a full stall. If the resistance is 0.6ohms your max draw is 21A on a fully charged 3s lipo