Re: How big of a starting battery do I really need?
Consider that the 15 hp engine you use is a 'marinized' garden tractor engine. Like MS says above, a U1 will do it.
Google this : 15 hp diesel engine starting battery
it will take you to a link (too long to post) that is for marine starting battery calculations.
Bottom line - 15 hp - 420 CCA is sufficient to start you 6 times with no charging per The Marine Electrical and Electronics Bible
Lessons learned are opportunities earned.