Music room dimensions

In another life I would like to have a dedicated music room. Any suggestions as to size of the room? Square or rectangular? Ceiling height?
i realize the type of speakers play an important role ( planer or box ). I have Magnepan speakers that need to be placed away from the wall for best sound. I also have two subs.
just looking for a general rule of thumb.
Why put it off for "another life"? What's wrong with this life???
It's your ears, let them tell you what they like.

More stuff to consider

I use the speaker calculator, and hear good results. YMMV. Others may try it and think...meh
Golden ratio. Length + width divided by length = 1.618.
20’ x 14’ would be a starting point.

@elf1, Maggies (and all other planars) benefit from being as far from the wall behind them as possible (to avoid the front-and-back waves from creating comb filtering), 5’ being a good objective (to create a 10ms delay between the arrival times at your ears of the front and back waves). Distance from the side walls isn’t nearly as critical as it is with non-dipoles, as the length of the Maggie drivers makes them a line-source, and the figure-of-8 radiation pattern of dipoles creates a null at either side of the panel. Magnepan’s Wendell Diller has been touring the U.S. with the new 30.7, and he has been positioning the bass panels as close as a foot from the side walls. If you DO place the panels that close to the side walls, it’s a good idea to put the tweeter drivers on the "inside".

As far as room width, the further apart you can get the two panels, the further from them can you sit and still get good soundstaging and image spread. In a 14’ wide room, if placed a foot from the side walls, a pair of 2’ wide 3.7i Maggies will then have 8’ between the inside edges of the two panels---just barely enough if you want to sit about 8-10’ from them. The 1.7i is 19" wide, so the distance between them would be just under 9’. If you want to have them further apart (perhaps to sit further from them), or further from the side walls, you need a wider room.

And then there is room length. It’s great to be able to sit as far from the wall behind you as the speakers are from their wall, but that adds to the length needed. So, 5’ (speakers from wall) + 5’ (listening position from wall) + 8’ (distance from speakers to listening position) adds up to a minimum of 18’.

All the above is why 14’ W x 18’ L is considered about the minimum optimum size for a pair of Maggies or other planar loudspeaker. Height is of concern only in terms of the room’s eigenmodes---the standing waves created in all rooms by their dimensions. The "Golden Ratio" dictates a 1/1.6/2.6 dimension relationship for least objectionable modes, which of course translates to a room 10’ high, 16’ wide, and 26’ long. Few have that luxury! With the more common 8’ height ceiling, the width is only 12.8’, insufficient. I one time had a room with 9’ ceilings, and loved it. If I were to build a room, that’s the height I would use. 9’ x 14.4’ x 23.4’.