How large of a speaker do I need

Budget? Decor considerations?

As to your question about bass, if anything, the open end of the room will help longer (i.e., lower bass) soundwaves form. You are going to have a serious resonant node because of the 15' square dimension, however. That's going to be your biggest concern.

15'x15' is not particularly big. You could easily fill that with 50-100 watts/channel and speakers of 90dB sensitivity or above. Something like an Onkyo A-9555 or Cambridge 640A integrated and a pair of PSB Image T45 or T55s. Or if you have more money to spend, a Nuforce or Bryston integrated and a pair of Totem Hawks or Sttaffs, Paradigm Reference Signature S6, or similar. Paradigm's Monitor 11's sensitivity would be like tripling your power (claimed 97dB sensitivity in-room).

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.

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.