From my experience I would recommend using a 4" computer fan for outtake. I bought one online that was ready for AC power so I didn't have to fumble with the wires and risk making a mistake. I have a 9"x3" rectangle for passive intake and they seem to work well together.
For smell, you can either decide to spend more money and make a mini carbon filter system with ducting attached to your outtake hole leading to a carbon filter system on top of your grow box which pulls the air through and recirculates it either outside or back into your box, or do what I do and buy some Ona blocks and make an "Ona Bucket". This of course depends on how many plants you will have and how stinky your genetics are.
In mid-late flowering right now, I have 2 small Ona blocks in my growroom along with some other cheaper odor-absorbing imitation products I bought at a department store. The smell is virtually non-existent even with my growroom opened up for 12 hours. All I can smell is citrus and fresh linen and I harvest in 2-3 weeks!
Not sure if it's frowned upon to post links to other forums, but I found this DIY to be very helpful for the Ona Bucket, and everyone on this forum seemed to have great experiences with it:
During flowering, my sativa dominant stretched 2-3 times it's vegatative height, but my indica only stretched about 1.5 times her vegetative height.
Let us know how it goes constructing your box, I wish I had spent more time in the "planning stage" for my Rubbermaid setup on the Micro Grow page, but luckily I had some help from forums and friends!
Peace,
The Wiz