Are you talking about a built-in fixed system, or a portable handheld extinguisher? If built in, Fireboy has documentation with tables showing cubic foot coverage for various size canisters. So you will need to measure your equipment space approximate L x W x H.
Keep in mind that a fixed system will not function unless you also install an automatic shutdown for your engine(s), generator, and vent fans. Without it, all the fire suppressant just gets blown out of the boat before it can put out a fire.
For the shut down system I would avoid the SeaFire gray box system at all costs. In my judgement, it's a defective design with a single point of failure in the internal power supply that can and does completely disable the boat, including the override function that is designed to get the boat running again when the system activates. I know countless boats that have become disabled (no engines, no generator, just silence) because of this, including my own. Fireboy makes a solid device that is fit for purpose.