I think a good starting point in almost any room is a dozen 4" 2'x4' traps (or thereabouts).
But John, that's what you recommended to the guy in the 6'x7' room also.
Yes - I did. The reason for that is that you're in a room with a pair of speakers that are operating at a typical level and putting out a typical SPL. There's a certain amount of energy that needs to be absorbed from that amount of energy being put out by those speakers. If the room is tiny or medium or large, the amount of energy that needs to be absorbed is still similar. In a smaller room, that energy is contained in a smaller space - That's why it's typically better to "over-treat" (if that's even really possible) the space.
In your case, you have 3 dimensions all divisible by 2 (so is my space) so the treatment is even more vital (my space has the equivalent of around 35 traps).
Lemee throw in a visual... Say you have 10,000 gallons of water (low end) and enough sponges (bass traps) to collect 2500 gallons. If you're in a room that will hold 20,000 gallons, the room will be half full of water and the sponges will capture half of that - heights being equal, figure you're going to be up to your thighs in water. The room is still going to be 1/4 full of water. If you're in a room that holds 60,000 gallons, after those same sponges are introduced, your ankles might be wet. Same amount of water, same amount of sponges - but in a larger area, there is less of a water problem.
A similar thing will apply with broadband trapping -- The smaller the space, the more danger of "drowning" in low end.