32" x 18" top & bottom, 20" sides (trapezoidal)
I don't use it often when playing live, only for big shows, and then I stick it over by the drummer's hi-hat, and stand on it during bass solos (when I drop the groove to go off on my own, so I can better stay on with him). I mostly use it for sessions when I'm tracking live with a drummer.
I wish it were a little bigger but honestly, it's big enough that when you're standing on it, you can rock back and forth and move a bit and you don't have to worry about it. If you play with a pedalboard, you're probably used to keeping within a few square feet anyway and it's not a problem. I especially like to put one foot up on top of the driver housing, because you can REALLY feel the thump when you do that (kind of like putting your foot against the kick drum head). It feels like you and the drummer are sharing a brain.
Bass Boy:
Cool, I need to convince one of the drummers I play with that we need to try this thing.
You can also run your bass through it, which is a real trip - it's hard to explain - you can actually FEEL the pitches, but it's silent. It's not something you can really "get" if you've never felt it... you can tell if you are out of tune, even though you can't actually hear the notes, because it vibrates your whole body at the correct frequency to what you're playing, and you can also easily tell slap from fingerstyle, etc. It's so much fun!