What size cranking battery size is required for a 36 HP Prodrive?

Size as in battery group or cranking amps?

If the latter, as many as you can afford. I used to use a marine battery with about 450. Now I'm using an automotive with 1,000 and it starts a lot faster, especially in very cold weather.

i have a optima yellow top and have never had a problem with my battery going dead.. i have ran two 100 watt lights frogging off it as well

I've run the U1R, garden tractor battery, for years without any problems. I know, I know it only has 230-300 cranking amps, but it's a lot less expensive and the only accessories I run is GPS, depth finder and marine radio. There is no need for lights when I'm out. If I go on a long trip, 300-400 miles or more and stay out for a week or so I'll throw in an extra battery just in case I do something stupid like leave a key on.

1000 CCA battery from Academy- works good, lasts a long time.

Odyssey AGM 1750 cranking amps normal, over 900 at zero degrees

Obviously you have a lot of options, comes down to price. If you have a CDI motor w/ 50A stator that charges at idle, you could probably get away with a little less battery- especially if your lights are LED. Size also depends on your winter low temps and the accessories you run off the cranking battery.