Floats work great in the Dakotas for some reason (probably a spring thing) and almost not at all here in MN where I'm at (no spring season).
The only guys I know that have had any luck with them here in MN are guys who use them under lake docks for nuisance rats, but by the time season starts those docks get pulled in...soooo.
Try a couple before building 100 of them is my advice.
As far as dimensions, you don't want the float much wider than the trap. I've watched rats climb all over floats (and traps) without getting caught. Rats hug the edge of a float most times and will walk on the outer rails.
Watch a rat walk all over your freshly tuned trap and you'll re think how little pan tension is necessary to catch the bulk of your rats. It's an eye opening experience. Its not how many rats you are catching but how many rats you are missing that's the key. It's more fun when they walk over your competitions traps, though