If your growing with CFL's alone, opening the closet door should be sufficient. However, if you want any sort of decent yield, you'll incorporate an HID lamp, and for that, ventilation becomes a must. Before designing a ventilation system, take a step back and consider its purpose. When it comes to growing plants, the way i see it, a ventilation system serves 2 major puposes; 1) To maintain a good environment by replacing hot, depleted air, with cool, fresh air. This maintains a stable temperature and provides a constant supply of CO2 to the plants. 2) To facilitate evaporation of waste products from the plants. Plants breathe through their leaves, expelling toxins through them to be evaporated. A lack of moving air causes the stomata to clog (because the toxins arent going anywhere) which in turn means the plants are absorbing less CO2. No CO2, no growth. Very simple. Alright, rounding up for ease of calculation, lets assume your grow space is 6'x2'x2'. This gives us a total cubic footage of 72'. For a space like that a 100CFM(cubic feet/minute) squirrel cage fan would do the trick for ya. They sell usually for around 100-150$ at hydroponics stores. Some people install intake fans aswell, personally i dont as without one, your exhaust system will maintain a negative pressure on the room, which is a good thing from an odour control standpoint. On top of that if odour is an issue for you, you could always pump your exhaust through a carbon scrubber, ionizer, or ozone generator. Beyond your intake/exhaust system, you're going to want a smaller circulatory sub-system. This will consist of a series of small oscillating fans (i use 6" computer fans, they cost me about 5$ a piece) to maintain good air circulation within your grow room. And there you go! Hope this offers a bit of insight, and best of luck with your grow!
Cheers, Sergeant Stoner