If you go to the very top of this forum you will find a thread called something like frequently asked questions about float flying - done nicely by Jim Casey - Jim is very informative and will not steer you in the wrong direction. The rule of thumb is to measure from the rudder hinge line to the back of your prop ( front of thrust washer ). Take 75-80% of that distance and this will give you the lenght of floats needed. I put 40 size floats on a beefed up Sig Kadet Senior yes they were a bit shorter than called for , the plane sat a bit deeper in the water than a 40 size trainer, but I had no problems with this set up.