The right room.

What is the optimal size room for two channel? That is for medium size floor standers or monitors. Will not be going to horns or planars which probably need more breathing room. Something that would be ideal for merlins or the likes of usher's BE- 718.
Based on my limited experience, I'd prefer a room size of about 14ft wide x 23ft. deep X 10 ft high ceiling for medium to large floorstanding speakers placed far out from the back short wall.

These dimensions should suffice for perhaps all but the largest of speakers providing a deep, wide, and tall soundstage with proper speaker placement.

And with a room of these dimensions there should be enough space to play with speaker placement and listening chair placement to minimize most any low frequency suckouts and overloads.

From my experience, Stehno's suggestion is accurate.

All room dimension rules are made to be broken but going with a long rectangle as he suggests will give your speakers room to breath and allow them to reproduce even the deepest bass passages.
Yeaaaaaaaaaaaaa my room is 14X23 but sadly I have low drop deiling in basement (7ft) but I run speakers with very little Vertical dispersion. If I had a choice I would extend room size to more like 17X26 or so with a high ceiling. My point of view on room size is for dual use to allow for a larger Projection screen then I can fit now.
Being a contrarian by nature, I have to disagree. I think any room can work to some degree if you work it. There is always a solution when one considers objects, surfaces, placement, and listening area. Experiment like crazy. Break every rule. Know that someone with real talent can find the answer, and practice the craft until you become better at getting close to that answer. And if all else fails: hire someone with the skill and the ear!
Just out of curiosity, I know that too small is detrimental as I have that situation now and there is not enough room for my speakers to breathe. But as the room gets bigger is there a risk of losing bass? I mean is it possible to have to big in relationship to speakers. I am quite aware that bigger room will mean needs bigger speakers. But that may mean using speakers with multiple drivers serving the same range. And I prefer two-ways with one driver per way, or if a three-way but again one driver per range. Any thoughts??? Can speakers like scaena sound coherent, they got excellent reviews?
One of my best friends has a room that's about 25 wide, 38 long and 12-14 high. A very large space, and his reference speaker is a medium large three way with 13" woofer.

The sound is excellent, although perhaps too low of an acoustical output for my taste with his current power. The new amps he has on order will likely bring this system into the realm of best of the best.

Point I'm making is larger is better, unless you have too small a speaker or too little power to get the room moving.

I agree with your comment about two way speakers. With only one crossover point they get closer to the ideal of NO crossover point. Your question of coherency is valid, fewer crossover points are not a guarantee of phase accuracy but are definitely a big factor.
26.2'x 16.2'x 10'