What is ideal dimension size for a home listening room?

If you have the opportunity to build a room, and you have the space, make the room as big as your budgeT and physical constraints will allow. I am in a dual purpose custom built listening room that is used mainly for audio. But, wife wanted movies in the cavern, so I had to accommodate the home theater for the wife. I learned noise mitigation techniques in the Army and incorporated that knowledge into the design. If your building out a room, then adding acoustical elements such as Roxul Safe and Sound sound insulation in the walls, and perhaps increasing drywall width, by doubling up, or buying Quietrock Sheetrock will aid in reducing noise from escaping the room. You can also get a company to spray foam the joists in the ceiling of your room to further reduce noise leakage. Generally speaking,  it’s easier to tame a large rooms acoustical anomalies than a small room. It can be done, but the audio presentation will be vastly different in a smaller space. If your room is a dual purpose room like mine, move the AV rack out of the room to free up space and reduce heat.  

I just watched a Dennis Foley Acoustic Fields video, in which he makes his case against the claims made for Green Glue. I can’t speak to that product, but can to another wall damping material: Wall Damp, made by ASC, Acoustic Sciences Corp., the Tube Trap company. It is a viscoelastic material made to be installed between double layers of sheetrock. I heard a room so constructed employing Wall Damp, that of Audiogon member folkfreak. I rapped my knuckles on the walls of his room, and it was like stone: very close to absolutely non-resonant.

I wonder how cost effective Wall Damp is to Quietrock and if it compares to the performance parameters of Quietrock Sheetrock. I think on the low end, Quietrock double layer 5/8 EZ snap is roughly $60 per 4x8 sheet. Using Quietrock 530 is about $110 per sheet average. How much does Wall Damp cost per sheet or roll? Wall Damp is certainly a viable solution to sound mitigation if it is not too cost prohibitive and less than the cost of using Quietrock.

18x28 is smokin dude

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.