Whether you classify your room as large or medium really doesn't matter. Choosing an appropriately sized speaker depends not only on room dimensions but also openings to other rooms, wall/ceiling construction, listening volume preference and musical tastes. You also have to take into account that some speakers are designed to work best in real rooms and others are simply boxes with drivers that measure well in anechoic chambers.
Being that there are so many variables, you can really only try an individual speaker in your room and see if it works. I'm not saying that the advice offered by others above is wrong, but what they said will unnecessarily limit your possibilities. I've heard very large speakers perform wonderfully in rooms much smaller than yours. I've also heard small speakers sound great in really large rooms. I've also heard mediocre to bad sound in each situation. This subject is too complex for easy generalization.
Being that there are so many variables, you can really only try an individual speaker in your room and see if it works. I'm not saying that the advice offered by others above is wrong, but what they said will unnecessarily limit your possibilities. I've heard very large speakers perform wonderfully in rooms much smaller than yours. I've also heard small speakers sound great in really large rooms. I've also heard mediocre to bad sound in each situation. This subject is too complex for easy generalization.