What Size Floats do I need? 

The commonly accepted guideline is the length of DUAL  floats should be 75-80% of the length of the

 plane.  Measure from the propeller drive washer to the rudder hinge line.  Multiply that distance by

0.75 to 0.8.  The closest available float length should be about right. 

Some floats are the right length but they are still too big.  Measure or calculate the weight of your plane

 in ounces. Each float should support  the entire weight of the plane.   Lots of math here but it boils

 down to approximately:

Width=SQRT (2.5 Weight(oz)/Length(in).    So a 7-pound .40-size trainer with 32” floats should need floats only about

3 inches wide.  Those might look funny, but if you have the choice of 4” floats or 6” floats, get the 4” floats.

What about a single float? 

Uncommon, but  some planes have only one, centrally mounted float . 

The length of a single float should be 100% as long as the plane that carries it. 

Use the equation above and double the width.