What's ideal size for a listening room?

This question is too generic for any kind of specific answer(s).  For example, first and foremost, what type of sound system is being used?  Are the rooms in question strictly rectangular or do they have odd kitty-corners or odd shapes?   Doors, arches or other openings?  Acoustical properties (e.g.  Typical drywall construction?  Insulation?  Solid cement, wood or other type walls & ceilings?  Windows?  Curtains?  Drapes?  Etc.)?  All of these are factors.

If it's not too much of a PAIN, set up your rig in both and let your ears tell you which is best.  Generally, my guess is the room with the higher ceiling would be your best bet.

@emergingsoul   You mention that the larger room is "open."  If it is open on one long wall and not the other (as is my listening room), you’re apt to get the best results with panel speakers--they lessen side reflections, compared to box speakers.  If you have a full, hard corner for just one speaker, the bass will be stronger on that side, and this is worse with box speakers.  Luckily, some panel speakers bring other benefits--their taller height can allow for better vertical imaging, and they are good at creating soundstage depth.

A larger room is likely to be better at handling louder volume.  So if you want to listen at realistic volumes to rock or orchestral music, the larger the room, the better.

Larger and more open sounds like the better choice. "Filling" is a very mixed bag. The smaller the room the more problems are reflections. I have a big Z shaped room(s). about 35 by 40. It is ideal. There are hall ways and asymmetries everywhere. My dealer says it is the very best room he has heard save one, and he has heard hundreds... many purpose built. The room itself... a leg of the Z is 25' by 20'. I have not trouble filling it... because I have a 9' triangle listening area with the back wall being about 4' back from the speakers. No reflections from the rear and none from the side. It sounds great. 

Go for the bigger room unless it prevents you from turning it up to what ever volume you want because it will disturb someone. This is more important. Any room with enough work can become great. 

The 8’ ceiling for the win every time.