Look at the pictures of full scale aircraft on floats... the floats are very large compared to the aircraft. The 80% of fuselage length formula works. Trust it.

If the floats are shrter, yes they might work. problem s, you lose some stability when on the water. It makes float placement related to CG MUCH more critical if you go with shorter floats. It is better to have them too long than too short.

You may also see that the front of the float is forward of the prop... this can seem strange, but remember the float's center of boyancy must be under the aircraft CG or the plane will not ride correctly in the water. The CG of the aircraft is generally about 25% of the fuselage lenght from the nose. The float center of boyancy is generally 40% from the front. (wider front than rear) It is OK to have the plane ride a little tail-low with empty fuel... Never OK to ride nose low with full fuel.

Rigging your first float plane... follow the numbers, they work.