Honestly, I've never seen a breaker placed on the positive main between the starter and the battery, in either marine or automotive applications. However, should you want to do it, here are the steps you need to follow:
1) determine guage of the wire running from your starter to your battery. The size of this wire will determine the maximum amperage you can run over that cable.
2) Using a amp meter (clamp on style would work well in this instance), determine what your current draw is when you start the motor. A cold start will draw more amperage than a warm start.
If you are currently drawing more amperage than what the cable is rated for (you can google this), you need to upgrade your cables ASAP. If not, you can now place a appropriately sized breaker in the circuit.
However, please read the following:
http://www.jamestowndistributors.com...t.do?docId=108
ABYC Standards exempt wiring between the batteries, the main battery switch and the starter motor. These wires can also burn if too much current flows through them, however, it is often not practical to provide such protection. (quoted from above linked article)
Personally, I see this as adding unnecessary complexity and points of failure to your system.
Jim
