What makes a speaker too big for a given room?

In general the more drivers and the farther apart the drivers are they are the further back you have to sit for them to integrate. Also speakers with a lot of bass may overwhelm a smell room. But remember, this is a matter of taste. I use to tease a friend that his Tannoy GRFs were the world's largest headphones as he sat right on top of these very large speakers.The advantage of using seperate subs is that you can tune to bass to fit the room more easily. FOR MOST PEOPLE monitor speakers , with or without subs, will work better in a small room but YMMV.

Money.

What Stanwall said. If you cant get far enough away, then you might hear each driver seperately instead of as one wavefront. Some time aligned speakers are designed with a certain listening distance in mind. Also, bass notes have a long wavelength, some exceeding 20 feet, so a 25hz note might not even create a single wavelength before it bounces off the rear wall and back on to itself.

Small rooms are better suited to stand mount monitors. I use a NHT Classic 3 in a 11 x 11 room with good success.