Intake & Exhaust Size?

I'm going to start my first grow indoors, but i have a few questions first. The size of my grow box will be 3x3x6. I'll be using a switchable ballast, 400w HPS for flowering and a 250w MH for Veg. What i need to know is, what size should my intake holes be and what size should my exhaust fan be? Thanks.

I think this is a CFM (cubic feet per minute) question.

If you want to recirculate all the air, then its easy. A fan will be rated by CFM.

For example, if you have 1000 cf. And want to recirculate in 5 minutes, then you need something that will have a CFM rating of 200.

If your asking what size of vent pipe to use, 4" should be enough. 6" is for 10,000s of cubic feet.

Side note: Fans have a linear feet rating also. Most dryers for instance have 80 lf rating meaning they will wear out faster if you pipe more than 80 feet. A 90 degree corner counts as 20 feet and so on.

Side note: I see alot of grow rooms pics using flexible heater pipe. White PVC sch 80 is much easier to work with long term and is inexpensive.