The outside dimensions on the supports are 17"x27". I ended up assembling the stand according to the directions, then measured the interior space and built the frame to fit as snugly as possible. I ended up using a hammer to tap them into place but that stand is completely solid. Even with the tank full of water, along with 100lbs of PFS and 150lbs of lava rock there is absolutely no bowing in the center...now if I could only find a matching canopy for it.