The armoredplanet website is pretty neat.

As far as spacing your floats, about 25% of the wingspan seems to be the number that people throw out. Hang your floats on the existing landing gear for a starting point.

Primarily float spacing determines resistance to tipping in a crosswind. If you have a low wing, you can get by with less float spacing. With a high wing and dihedral, you need more. The float spacing on my Senior Telemaster is too narrow by that rule, and it is a Little tippy in a crosswind, but not so bad that I feel the need to change anything.

If the floats are spaced too wide, I suppose it would start to hurt stability on the water and the plane would tend to yaw about as one float has more drag than another. This is a highly rare occurrence, and yaw is usually caused by poor rigging or the water rudder hanging too low.

If you have a specific handling problem then you'll know how you should adjust the float mounting from the problem you have.