I'm trying to set up my magic space, which will be a 2x2x5.2, with a 400w hps bulb.
My question is: will i be able to cook bacon there, or it will be ok, and if so, with how much ventilation do i need?
OK, I get it now: "cook bacon" as in tiny tent too hot.
Yes, a 400 watt lamp in a small tent is potentially a problem, but yes, with good air exchange you can carry away enough of the heat to make it manageable.
If it was me, I think I'd consider getting a "cool tube" type reflector in which cooling air blows right over the bulb and out of the tent. That should work well. A good carbon filter will help to contain the bacon smell.