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.

Smaller room has bigger possibilities for bigger sound.
Headphones have the biggest possible sound you can get.