Hi Bones,
My advice - go one or two sizes bigger than you think (same as buying a TV). You'll fill it up, and find lots of spots for stuff.
We have the bigger-than-standard box on the front of ours, and it's wonderful. When we bought it I thought there's no way we could fill it. Now it holds the porta-potty, the tarps and canvas walls, normal pegs, sand pegs, ground sheets, kitchen bucket, mozzie coils, tie-down straps, tools (strange, I know), yadda yadda yadda. I'm thinking of installing some tie-downs so it can take the gennie as well. If I had my time again, I'd go the even bigger box, just to give a bit more flexibility.
Much easier and cheaper now to buy the bigger size (even if it's more $$$) than to upgrade later. Just cause you have it, doesn't mean you have to fill it.
Not sure on the dimensions of ours, but here's a photo:
Good luck!
Matto