What is would be a small listening room size?

12'x17' is definitely a small room.

15'x20' would be a medium size room.

IMO depends on if a home, or an apt. And if a living room, or a basement.
A small apt room is 10' by 12'
A medium apt room is 11' by 18'
a large apt room is 14' by 24'
I know of luxury apts with 22' by 40' living rooms... That would be giant sized room.
(my apt listening area is 11.5' by 28'.. with an intrusion for a divider behind the listening area)

In a modern home i would expect rooms to be larger than in an apartment.
In an old home, not remodeled, the rooms will be more apt size.
Open plan home may have huge areas.
And way more than 8' ceiling depending
And of course the big homes of the wealthy may also have huge listening areas, though i know some with a small side room for a listening room.. (WAF issues)

Basements are all over the place, due to construction. Most have a lower than 8' ceiling. And if you live in a flood plain, not a good place for a stereo, sump pump or not...

http://forum.audiogon.com/cgi-bin/fr.pl?ymisc&1239284130&openmine&zzWinoguy17&4&5#Winoguy17

Medium.

I use the terms "small", "medium", etc. to reflect a room's suitability for large, high end speakers. Many large, full-range systems produce enough bass to overwhelm a typical 12 x 17 x 7.5 space. Many large planars will physically dominate such a space. In my book, a large listening room can accomodate all but the lunatic fringe loudspeakers.....which need very large ("huge"?, "palatial"?) rooms. IMO, the space the OP describes will probably be too small for best results with a LOT of high end speakers that will work well in large rooms.

That said, when I look at the "systems" links for some 'goners, I see some very large speakers stuffed pretty tightly into smallish spaces, so - evidently - some other might use similar critera and reach a different conclusion.

Marty

Your room is comparable in size to mine. (around 14X19X8)
I consider our rooms small. Better suited for small to medium floorstanders.