Standard rule is that you need double the intake size, of your exhaust fan. So, if you were running a 6" exhaust fan, you would want the equivalent of 2x6" intake ports. If you have a 2nd fan for the intake (known as an "active intake"), then your intake and exhaust can be the same size.
You want your walls to suck in a little. If your room isn't blacked out, and you can't use the micro screens at the bottom of the tent, you can take some ducting and use it on one of the bottom 6" intake ports. Wrap it around itself a couple times to make sure no light makes it in the tent. You'll want a filter on that ducting, so use a nylon stockings/panty hose, and stretch it over the end of he duct. Works awesome as a filter!