So I went into Radio Shack and picked up this 6"x4"x2" project box and threw all of my electronics in it. There are 2 5Ah 3S batteries in there. It barely fits! Will I have any issue with my Xerun ESC overheating with no vent holes?
I would definitely put vent holes. Better to be safe then sorry.
Definitely need to ventilate any ESC that comes with a fan. They put fans on those things so they can push the performance envelope of the less expensive parts they use.