It depends on the size. However, if you've never had to use a fire extinguisher to put out a real fire, you will be surprised to learn how fast those average sized extinguishers you see under people's tires at car shows last when needed in an emergency. You get about 10 seconds of use from those. If you can't put out the fire in that amount of time, you're in deep sheep dip.
In the shop you should have more than one LARGE extinguisher - like the 2 or 3 foot tall ones.
