I am trying to find a general rule regarding float width and height. I have found all kinds of info on length and step placement. I am planning to cut foam floats for my ag-cat. It weighs 3 pounds and the float length would be around 30" (at 75% fuse length), but I may make them 32".

Here's an online interactive float dimension calculator.
http://www.armoredplanet.com/hcfc/floats.html
it'll give you width, as well as the other critical dimensions.

if in doubt, it's hard to make the floats too big.

Thanks Jim
That is a handy program. I would agree, it is hard to make them too big, but it is nice to have a starting point.

I have noticed a lot of full scale float planes have some lateral area added to the tail, either under the aft fuse or under the horizontal stab. Is there any way to predict how much area is needed? Or is it not really done on models.

Usually, R/C models have bigger tailfins than their full-size counterparts so additional fin area is not needed. Scale Cubs have a ventral fin. Some scale beavers have finlets on the Horiz stab (a Kenmore Air modification. Also, R/C models usually have less dihedral than full-scale planes, so the vert stab is not outvoted by the vertical component of the wing area, so they're not as likely to get into "dutch roll".

I'm sure there is a method of calculating it, but not at my disposal.

The vert stab is not much larger but the rudder has been enlarged significantly. I will just have to try it. Again, thanks for the help.
Ben

The rudder is just part of the vertical stabilizer ....until it is deflected.

RC airplanes that need more stabilizer area to handle floats may exist, but they are few and far between.

ok