The recommended length that a float should measure is aproximately 75% of the overall length of the aircraft.A little longer won't really matter as long as your not killing it with a ton of extra weight also .The wingspan of the model is not really part of the formula when trying to figure out float dimensions for an aircraft.If that was the case,you wouldn't have clipped wing Cub's flying off water with floats.A larger wing generates more lift at slower speeds thus making it quicker and easier to get off the water.However,the larger wingspan model also weighs more than its short winged cousin causing the floats to submerge more and raising the water line further up on the sides of the float.The hardest part of float flying,is getting the floats to rise and plane on top of the water.This allows the plane to pick up enough speed so that finally the aircrafts wing can begin to generate enough lift to fly it off of the water.Without speed ,you aren't generating much wing lift.With the floats riding higher up out of the water,less power and effort is required to get the floats to plane,which means you are going to obtain your lift off speed much faster also.A float plane with good floatation is very important also when it comes to success in float flying.It's not just a matter of float size and length.Once you have both models(standard wing,short wing) up on step and planeing,the standard wing will lift off at a slower speed than the clipped wing version due to its increased wing area.The clipped wing aircraft needs to plane on top of the water much faster in order for it to generate similiar wing lift of the full sized version.This means longer take off runs.Good float floatation and a model with a lighter wing loading will reduce the distance of the take off run.What you want to figure out is what your wing loading will be with the added floats on your plane.The higher the wing loading,the harder and trickier it will be to get it off the water and to fly it.A well designed float makes this task a heck of a lot easier.Happy flying!