You can get whatever size you want, as long as it fits in the trays (if your boat has them, I imagine it does).
As another person said, it's RC (reserve capacity) that you want for trolling motor batteries, not cranking amps. Reserve capacity and amp hours are related. If one battery measures in RC and another in amp hours, you can compare the two capacities by multiplying RC by 0.42 to get to amp hours.
Reserve capacity is "the number of minutes a fully charged battery at 80 degrees F is discharged at 25 amps before the voltage falls below 10.5 volts."