RE: Float Size?
Float length: 75-80% of the length of the plane from the prop washer to the rudder hinge line.
Alternate rule: nose of float is 1/3 of prop diameter in front of the prop. Step under the 40% Chord point. Step is in the center of the float.
Step of float: under the 40% Mean Aerodynamic Chord point.
Hang the floats a little nose-down (3-5 degrees) so the wing will be able to make lift while the float is running horizontal along the surface of the water. (Top of float level-Wing at 3-5 degrees positive incidence)
Prop arc should clear top of float by 2".
Track of float: 25% of wingspan
Mount the floats rigidly. 2 aluminum landing gears and spreader rods between work very nicely.
Water rudder(s): should hang down no farther than the depth of the step.
Balance the plane in the same place that it would balance with wheels.
You don't need more engine, but dropping the prop pitch an inch is sometimes good. (use 11-5 instead of 10-6)
Mount your switch internally using a pushrod to actuate it.
Don't mount any of the electronics either at the top or the bottom of the fuselage....any water can slosh around without dunking the circuit boards, even if the airplane is inverted.
Seal the inside of the fuselage with a coat of dope/paint whatever to protect the wood.
Use styrofoam floats instead of hollow floats. Hollow floats are good for storing water.
We're here to help. You're gonna have fun.