Within reason, I don't know if there is a too big for the room. As Alan Lofft has said in other posts on the forum, speakers move air. My non technical take on speaker size is that, a larger speaker in a small room will sound great because you don't have to turn it up as much to have the same impact as a smaller speaker played louder in the same space. Also, you will probably change residences at some point and you may be able able to afford a bigger space. If you have large speakers already, you won't have to upgrade.
As far as amp power versus sound level, a 200w amp is not twice as loud as a 100w amp; the volume to power gain ratio is logarithmic. For example, to play twice as loud an amp would have to have 10 times the power. What a more powerful amp will get you is more clean headroom. If you listen to a movie soundtrack or music with a lot of dynamics, a well designed more powerful amp will handle the peaks better (with less compression and distortion).
All things being equal, if you have the money to spare, to me, bigger is better.
Most of us have to compromise and balance what we can spend on the components that make up our systems (bigger speakers, versus better amp versus better source, etc.). Everyone is going to have a different take on this but I have always preferred tower speakers and higher powered amps even when I lived in a 12 x 12 room.