For clarity, I mean the configurations in the top images here. The top left is the most common configuration and supposedly most effective. Top right is okay but the filters aren't as effective in that setup. So I've been led to believe. A 5" kit should be fine for your tent size. I use a 4" in a slightly smaller tent.
none of those set ups look proper.
the image on the left is the most common, and is generally accepted as proper.
in both images the green line represents an external wall.