How large air ventilation hole/s?

[ALExanderH]

I'm making a box around my SCR control and I have a fan cooling the heatsink. I've drilled a few holes for the fan to suck air in but I guess I need some holes for it to get out of the box as well?
Does it matter where they are?
How large do they have to be?
How many?

I want the "out" ventilation to be minimal because I want the box to look nice, that's the only reason I'm using wood and not a regular dull gray electrical box

Yeah stupid questions but I want to get it right so the SCR won't overheat and I don't want to rip the box apart to add holes all the time

 

[mongoose33]

How about the same number of holes, same size?

What comes out must go in. Just have the hole locations on the sides or top so they're unobstructed.