A bow thruster draws a lot of power in short bursts. Therefore, I view the battery, total cable length, cable cross-section, fuse, and main switch as one system.
Start with the specific thruster model
You cannot safely choose a cable and fuse based on the boat's length alone. The manufacturer's electrical reference for the specific model specifies the current, recommended battery/CCA, fuse, and cable dimension.
Total cable length means there and back
Sleipner defines the total length as the entire current path from the battery's positive terminal, through the fuse and main switch, to the motor, and back to the battery's negative terminal. This is the length used when determining the cable cross-section from the manufacturer's table.
The battery must be able to deliver the power
A battery with many Ah is not necessarily a good thruster battery. The manufacturer also uses CCA and the specific load as part of the dimensioning. Low voltage at the motor results in lower RPM and less thrust.
Fuse and main switch
Sleipner recommends a fuse and main switch on the power cable close to the battery's positive terminal. The main switch must also be accessible so the system can be de-energized.
Long cable route or battery nearby?
A battery closer to the thruster can reduce cable length and voltage drop, but it also creates an extra battery that needs to be charged and secured correctly. Therefore, I calculate both solutions before choosing.
See bow and stern thrusters · See cables · See fuses