Depends also on the width (beam) of your floats. For that model I'd go for a pair of floats about 3" wide, or just over.
If your floats are too wide (say 5" wide each) then your model will be up on the plane at quite a low speed. It will be planing along, and bouncing from wave to wave, and bouncing up into the air, at far too low a speed to fly. So it will tend to bounce into the air, stall, and cartwheel into the water.
If you find this is the case, try using narrower but deeper (and/or longer) floats.
For a model of that weight you want floats which stick out ahead of the prop by about 10% of their length, with the step just aft of the aircraft CG.
The total length times max width times max depth of each float should be about 600 to give the required buoyancy.
Maybe something like 40" long by 3.25 wide by 4.6" deep
On the website
look under "Alasdair's Section
" for an article on float design and construction.