Don't talk about 200 rpm...that's gonna take me some time to achieve...... the reason my biggest is 68" is that I've got a long 2 mile climb/drag home, so if I can spin the 68 upto 30 mph on the way to work (down the hill), I'll be happy, as the car's slow me down to that, especially with the speed camera....
The 63" is for getting up a number of 1 in 10 plus gradients the other way when I go out training. All on the advice from my LBS who rides his fixed in the area - and he was a National Level Road Racer. If I find it too spinny, I'll change sprockets...