Art,
Not an easy thing to answer. The primary concern is the draw of the starter  
motor. The amps and the distance will determine the correct wire gauge. Your  
owners manual should tell you the draw from the starter motor. I would mount 
it  in a fused circuit with the circuit panel in the cabin somewhere. West 
Marine  will have charts available which will guide your decision. The wire needs 
to be  marine grade, which means the copper strands have been tinned to 
eliminate  corrosion.
 
Rummy