For that sized space definitely look into a 600w, its footprint is about 4 x 4.

As for your exhaust questions, the way you suggested is OK, but for that sized setup you don't really need a fan blowing air in. I'm not sure if a 4in. exhaust fan alone will cool the room enough, but you really need to look at the CFM's of those fans. There are some different formulas around online which will give you the required CFM's for your space and the light you decide on. There's one formula in here: http://boards.cannabis.com/indoor-gr...-progress.html

There are plenty of grow logs around also with people using 400w and 600w lights, and you'll see what kind of ventilation they're using.

Also, a cooltube will help a lot to keep temps down.