Class a is for "normal combustible" -----wood, paper etc.
Class b is for flamible liquids.
Class c is for "electrical" fires. This is usually a class a or b fire, with a live electrical wire involved--------electricity doesnt actually burn.
Class d is for metal fires
A typical extinguisher will be classed for a, b, and c. The numerical rating is a reference to the amount of fire that the extinguisher can put out. Bigger is better.
Extinguishers usually need to be checked annually, by a licensed service.
Stay away from pressurized water extinguishers for your hanger. They will cause a fuel fed fire to spread.
Halon, and CO2 are good in that they dont leave residue, but their effectiveness is reduced when used in the open, they work best when confined indoors.
For my money, the largest abc rated extinguisher you can handle/afford. Buy two, install one on each side of the hangar, that way unless the fire is really big, you can still get to one of the extinguishers. If the fire is that big, dont waste your time trying to put it out, just run........