Ordered me a trolling motor for my jon boat. I'ts a 40lb thrust. What size battery do I need?

Depends on how many hours you plan on running the TM between charges. Example is a MinnKota Endura 40# - will draw 42 amps at max thrust, 8 amps at min thrust, speeds 2-4 take 12, 15, and 21 amps respectively. So to run it at speed 3 for an hour takes 15Amp-Hours (AH).

 

So figure how many hours at what speed, add it all up, multiply by 1.5...and that's the minimum AH rating you should get for the deep-cycle battery. Hooking batteries in parallel add the AH rating together - so two 75AH batteries hooked negative-negative/positive-positive will give you 150AH.

 

I'd also suggest a Group 29 or Group 31...that's the physical size of the battery irrespective of the AH rating.

I've got two 100ah batteries connected and I can go up to 5-6 hours. But , it depends also on other factors. i.e. They'll make an electric truck soon. Great idea, but if you tow a trailer the time to discharge dramatically reduces. So, if  you're going against the wind or have a heavier boat it won't last as long.