What's ideal size for a listening room?

A room with these ratios is ideal:

Height - 1m

Width - 1.618m

Length - 2.618m

Extrapolate that to your room and the bigger room is best for. Also a larger room means the room modes are more spread out which makes room treatment less of a pain.

I'd go with the 15' x 20' with 8' ceiling room. While its dimensions don't perfectly align with the Golden Ratio, its larger size and higher ceiling will provide a much better starting point for achieving good sound, especially when combined with proper acoustic treatment (bass traps, absorption panels, diffusers) and careful placement of your speakers and listening position. @emergingsoul 

Both rooms have excellent dimensions. I like the larger one because of the 8 ft ceiling. It will take more power to fill it up. It will give you more room for speaker placement on the short wall which is optimum in a rectangle room. When you say it is more open, ideally in the back would be best. Open on either side would be difficult to deal with acoustically. Nice you have options. Have fun with it.

14' x 20' x 8' or some multiple thereof.