What is would be a small listening room size?

Size is in the eye of the beholder,three inches to some is eight !!! Oh, never mind.

8000 = 40x20x10h, no?

That seems to me a very large room, with a footprint comparable to many houses, in many parts of the country.

I might call your room medium; I think many speakers designed for smaller spaces would start to swim, and you could accommodate some larger room designs.

John

Your room is medium, small would be 10x10x8.

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...