How big is it?

Largely, room size has to do with the spkrs size. A room's dimensions are important (see cardas' site and immedia's) to minimise/ control standing waves.

Now, a BIG room is a relative matter -- no. The biggest I have seen/ listened in is ~2250'sq. Cheers

A 'tiny' room is a 'small bedroom' 8' by 10' tiny speakers.
A 'small' room is a 'small apartment' size 10' by 12' to 11'.
An 'average room' is 12' by 16'
a 'larger' room is 12' by 20' or 14' by 18'
a 'big' room would be 14' by 26' etc
a 'really big' room is 16' by 30'
a 'giant room' is 20' by 40'
anything bigger is the inside of a small concert hall!
Feel free to apply my definitions to your room.
My 'small' apartment main room is 11' by 32' (because the kitchen/dining area, which has only a low 1/2 wall partial divider, lies straight back of the long axis of the actual living room space... thus giving me a much larger acoustical space to listen in)
My Maggies are 6' from the back wall, and I sit about 8' to 10 ' from the speakers.

I would agree with Elizabeth and say your room is on the large size, so buy accordingly.