What is ideal dimension size for a home listening room?

There are no "ideal" room dimensions with a 7.5 ft ceiling.  Ceiling height does mean a lot, and the higher the ceiling the better up to about 12 ft.  Since you are stipulating a 7.5 ft ceiling, my guess is that you might be thinking about finishing an existing basement space.  If so, recognize that the ceiling is going to limit what you are going to be able to accomplish.  I couldn't find a favorable Bolt Area using a 7.5 ft height without making the room too small to be practical.  So you are going to have a real challenge dealing with room modes.

If (or hopefully when) I have the opportunity to add on a custom built room, I would start with a carpeted concrete floor and a ceiling over 10 ft.  I'd like the room to be fairly wide, something approaching 20 ft.  I'd like the room to be deep, approaching 30 ft.  If you start getting much smaller than these dimensions, you can have your work cut out for you and face limitations in selecting a speaker that will mate well with the room. I'd select dimensions that allow the room to fall within the Bolt Area and which exhibit a favorable Bonello distribution.  26.5 ft x 17.5 ft x 12 ft dimensions fall within the desirable Bolt Area, would provide a favorable Bonello distribution of modes, and have a relatively low Schroeder frequency.  Most of the calculated low frequency modes resulting from those dimensions are do not coincide well with the frequency of notes resulting from standard A = 440 Hz tuning of instruments.   As far as I can determine, this is about as good a scenario in a real world room as you can get. 

Such a room would offer the following additional advantages.  The  concrete flooring would greatly reduce problems associated with resonance being transferred back into the speakers and electronics as compared to plywood or OSB over 16" joists. Carpeting over concrete in conjunction with a high ceiling will substantially reduce problems with floor to ceiling slap echo and permits more flexibility in dialing in speaker to listening room position without having the room become too reverberant. 

My current room is 14 ft wide and 20 ft deep.  It is too narrow to accommodate more than one critical listener.   Another 3.5 ft would make a world of difference and reduce the intensity of side wall to side wall slap echo.   My speakers are full range and don't do all that well in near field listening.  So my listening position is only about 4 ft off the back wall, which creates an unfortunate low frequency null at the listening position due to out of phase cancelation coming from reflection off of the rear wall.  A deeper room allows one to move the listening position forward enough to push that rear wall cancelation to a low enough frequency that it becomes less important. 

@arion,  Good comments.  I've been intrigued by the use of non-parallel walls.  Conceptually, it solves a lot of problems if it is properly executed. I'm not aware of any commonly available mathematical models that provides predictions of modes in non-rectangular rooms, which would give me pause in committing to that approach.  

@audioquest4life , thanks for your kind words. Music is music indeed, and I enjoy music through my car stereo enough to know that sometimes you just need to turn off the audiophilia nervosa.  We all live in a real world and deal with compromises.  But a post like this invites dreaming, so dream I did.  It is akin to someone asking "What is the ideal 2 channel system?"  

I agree that near field listening can be wonderful.  In fact, I'd love to be able to build a system around that paradigm.  But if I were designing a new room, I would not want to get locked into a particular paradigm.  I'd want a room that would likely be as friendly to a near field paradigm using a monitor/dba approach as a more traditional full range system with the listening position further back.  I'd want a room that could work with panels, line arrays, point source, and horns.  

For the last 6 years, I've been "enjoying" my first dedicated listening room in which I'm using my first set of full range speakers.  It has been a challenge making it work, and I'm not completely there yet.  I've got the main issues addressed, so I'm getting close.  What remains is fine tuning, and it is not easy.  It takes a lot of critical listening to a lot of different recordings and an equal measure of critical thinking about what I am hearing. 

Most of the issues I've had to deal with were baked into the cake with room dimensions and construction choices that I had no control over since we bought an existing house.  Next time will be different.  I will either be going new construction or buying with the intent of building a music room as an add on.  

@docknow  55.8 ft in one dimension buys you the ability to sustain a 20Hz standing wave in one direction.  I'm not sure that would be on my top 10 list of goals for a good listening room.  Most of us are looking for what can be done with small rooms, that is, rooms that can reasonably be accommodated in a single family residential dwelling.  50-60 feet is about 2 times the length that most of us could reasonably execute.  

It is a tall order trying to get great low frequency reproduction outside of a concert hall.  Fortunately, there is not an enormous about of musical information at 20 Hz.  So we can do pretty well with most music in rooms that most homes can accommodate. 

@tomic601 ,  I ordered a swarm system about 6-8 weeks ago.  I'm hoping delivery will be soon.  The dba approach is, at least conceptually, a game changer.  We will see.

arion, thanks for the reply.  Your approach makes sense.  

I'm grateful for the online calculators we have, but we could certainly use some that are a bit more sophisticated.  It has been 48.5 years since I finished my last quarter of calculus, and I didn't use it enough professionally to maintain competence.  But I would think a physics graduate student could make quick work of generating a  calculator for non parallel walls at least for calculating axial modes.  The most difficult part might be figuring out how to present the output visually in a clear fashion.  

In some respects, I like the REW room calculator, because it allows you to introduce speaker and listening position into the model as well as room dimensions.  Unfortunately, my speakers have side firing woofers, so the model doesn't make the best predictions in my case.