Fire extinguishers are sold by what material you want to put out.
Wood, electrical, chemical, etc. if your workshop is just wood with the standard electrical supply. Something that stands about 3 ft high and is made for wood should serve you well.
Bear in mind H&S requires fire extinguishers to be wall mounted near exit ways and have the handle 1 metre off the ground. (or they did a dozen years ago) :roll:
I would recommend a six or preferably nine litre AFFF extinguisher sited as suggested on the escape route. Electrical fires are sorted by turning off the machine either at the socket or fuse board. The burning material is then safe to deal with. Google AFFF and you will find that it is also suitable for dealing with flammable liquids.
Extinguishers are cheap to buy providing you don't go to a firm describing themselves as providing fire safety services or extinguisher maintenance.
The most expensive mistake would be signing a service agreement of any kind.