Dunno about the size, but floating means the disks are not bolted solidly to the hub of the wheel, rather they 'float' on bobbins which allow the disks to move side-to-side slightly, meaning they run truer through the caliper, and therefor you suffer less distortion if the disks get too hot, and less friction if the manufaturing process hasn't aligned the disk / caliper too well.

Have a look at a floating setup and you'll see that there's a carrier, and a seperate disk, with the two parts held together by bobbins with a spring or wavey washer in there to cut down on rattling. (well, on most setups anyway)
_________________
MC21SP Plaything
BMW F800GS Bumblebee
Triumph 9551 Daytona Big boys toy
FJ1100 Sporting relic
GTS1000 oddball