The tank capacity is contained on the tank. There should be a metal plate with all the information. (Coast guard regulation)
Most likely on a boat of that size it would be around 16- 20 gallons????
Empty it out hen fill it up and see..........
Anything to do with the motor, transom assembly, outdrive, dash wiring, power trim and shift control are contained in mercury manuals or aftermarket manuals.
The boat manufacturer typically does not have any of this info even in a "owners manual"......I have seen some owners manuals that contain specific wiring info for there custom applications........such as winshied wipers, radios both listening and two way or maybe electronics.