Basic float selection requires the floats be about 75% the length of the fuselage. The step on the bottom of the float is in put at the CG of the plane or close to it. The floats are mounted 25-30% the width of the wing span. This is selection and mounting method is what I use on float planes, it works. Though I am sure someone can refine my method. Hopefully this is the info you were looking for.
Oh do not forget the water rudder. It will need to be hooked up to the control horn of the planes rudder via Golden Rod or flex cable. Try to get a water rudder that will kick up out of the watter as the speed builds up, a little goes a long way in the water.