what happens if the floats are too short?
If the floats are too short you will get away with it on smooth water and not notice a problem. When the water is choppy however you get problems with the prop clipping the waves, slowing the engine and kicking up spray.
You want a bit of float, minimum 10% to 12% or even up to 15% of the length, ahead of the prop so that they help to lift the prop clear of the waves. Jim Casey's rule of thumb probably works with most models, but mine works on any conventional model (i.e. engine at front, tail at back).
Make the float length at least two and a half times the Prop to CG distance.
Place the centre of the float at the aircraft CG.
Place the step on the float about 2 to 3% of float length aft of the CG.
Make the step depth at least 1% of float length.
Another tip is to make sure the floats are held parallel by tie bars front and rear.
Happy splashing
Alasdair