How large of a speaker do I need

the kg4 may be fine. they punch well beyond their weight. i would consider upgrading to klipsch cornwalls before anything else named, provided you enjoy the character of your current system.

In answer to your earlier question about losing bass energy, it's not an open ended room that will make the bass escape; that'll let the longer bass waves form and roll back and forth. What makes you lose bass is how porous and/or flexible the floor and walls are. You'll get better bass, for example, in a masonry room that contains the full integrity of the bass waves than on a suspended floor with wood frame walls that lets the bass energy leak out of the room.

Now that we know your needs better, it looks like you'll want to pick a speaker that gives you the holographic imaging you're looking for first and foremost. Since many of the speakers you mention have average to slightly lower sensitivity AND you like to listen loud, you *may* need a more powerful amplifier (but maybe not). You won't know until you try it out.

If you can find a speaker that images the way you like and has a sensitivity of 90dB or above, your 100 wpc should be plenty, even for loud listening. I have a pair of 91 dB sensitive speakers in an open architecture living space, and my 85 wpc (but very high current) amp is plenty to give me good volume for rock, big band, or large scale bombastic orchestra.

I second Jaybo - good advice.

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