I worked with a closet similar in dimensions to yours a couple times, in fact I think most growers have! The key to efficient ventilation is to have a powerful enough fan, but there are several other important factors to consider.
Assuming that you have an air coolable relfector, which I believe creates more efficient ventilation, you will probably need to move about 200-300cfm through a 600W light to keep heat from building up (just an estimate...)
If you are going to use a carbon filter, you must account for the resistance that it adds (filters range in resistance, bigger filter=more resistance)
If you are going to vent further than a few feet away from your closet, take into account the resistance that venting duct adds. Smaller diameter ducts add more resistance as well as any bends or turns in the ducting.
In general, I would recommend starting with a fan a little more powerful than you think you may need, it allows for additional expansion as well.
When venting, visualize the path that the air is going to take. You want to make sure that the air flows over the plants, and adding a small ocillating fan can help mix the air even better (I hung mine upside down by the cord and let it ocillate and swing around).
Go all-out and get a high quality inline fan or a squirrel cage blower that can move around 200-400cfm and I think you'll be satisfied with the results.
I also have used the actual walls themselves to vent sometimes. If your drywall is well sealed, you can use the wall as "ducting".
