Theyre usually measured in inches, so measure around the 'fattest' part of your calf and from your knee to the knobbly bit in your ankle with a tape measure.
once you have an idea, go to your local tackshop and try some on from that size. if they fit, all good, if not try an inch wider or an inch longer etc. you dont necessarily have to buy them from the tack shop once you know the size so can get them off the internet.
bear in mind that different makes and brands are different shapes so dont just go for the first ones you see. i can recommend the dublin ones for about £20 - they have kasted me a long time and are cheap. i do think they have shrunk slightly when i washed them, so bear that in mind.