5lbs minimum & that doesnt go far against much of a fire. I have a 10lbs in the bed of my truck. Only a 5 on the tractor & barely have room for that. Really need to put a 2nd on or figure out somewhere to mount a 10.
A:B:C are the common ones that take all reasonable fire types. They use powder that is rather corrosive after use though. So if you do use one, you have to clean things really well & even then are likely to have rust issues on steel & corrosion on wiring. In most cases it's better than having a charred husk of a vehicle.
CO2 isn't nasty for cleanup, but isnt as effective as the powder. Especially out in the open where the gas will blow away in the slightest breeze.
Water is good in some situations. Not as effective as the powder, bad for electrical, bad for oil, has freezing issues, etc.
For tractor use, only consider an extinguisher with a gauge & an all metal valve. Plastic is cheap & breaks, especially in the sun. They cost more but can be recharged & rebuilt. Plastic is disposable.