Hi I have a piper cub on floats it has a 76 in wingspan and weighs in at 11 lbs it's powered by a magnum .61 se with a 12x6 prop I was thinking of going to a 12x10 prop too much?

Yes a twelve x ten is way to much prop for your Magnum engine. If you have done a proper float installation with the step at a quarter inch behind the actual CG and the wing a few degrees positive to the floats top decks, your airplane will fly fine.

If you are still not happy with the 12x6 then try a 11x7.

John