[Giller]

I'm not sure the specific size - but having just done a hunt this past summer for a sub - I can tell you there aren't a lot of options out there that fit cleanly without some major modifications - or footwell loss.

I ended up with a self-powered Kenwood that fits very nicely under the passenger seat without any trouble. I have an upgraded head unit and no amp, so was a very simple set up.

If you have the bose MOST system, might be more complicated than a simple plug/play kenwood sub.