How large of a speaker do I need

I third Jaybo. Got Cornwalls myself in a similar room, though not with the square dimensions.

My experience is consistent with JohnnyB's. My room has one open wall in a
room that is app 24' wide and 14' from front wall to seating, with the large
open space behind the listener. It produces all kinds of problems, notably a
"hump" between 80hz and 130ish hz that peaks at +20db. The
room requires a pair of bass busters to tame this behavior. I also use EQ'd
subwoofers to deal with the mess further down in pitch. Open wall layouts
are unpredictable and you shouldn't presume inadequate bass reinforcement.

As to specific speaker recommendations, I'll always recommend subs (SVS is a
good place to look) and a Velodyne SMS-1 controller for room EQ. Add the
main speakers of choice, the little Magnepan MMGs come to mind and a brief
listen to PSB monitors also showed promise. A pair of 12" SVS subs with the
SMS-1 and either the Maggies or (I suspect) PSBs is a <$2500 speaker system
that will provide excellent bass in even a difficult room as well as fine results
over the remainder of the audible bandwidth.

Good Luck

Marty

Marty has a great idea here. It's *easy* to get good imaging from little stand-mounted speakers (and don't scrimp on the stands). Then get a good subwoofer and the Velodyne module. This will enable you to actively tune the bass to the room, both in amplitude and equalization. Meanwhile, your mini-monitors with their sculpted, minimal front baffle will provide maximum imaging with a minimum of baffle diffraction distortion and cabinet resonance.