Take the belt off the car, make a mark that you can easily see on the belt, and take a tape measure and measure the length in Centimeters, then count the number of ribs the belt has. Most belt manufactures use a standard part number system based on length to number their belts and the number of ribs it has, for example a 97.6 cm long belt with 6 ribs would be a 976K6 belt.
You really should not measure it. The auto parts stores have books that they can use to look up what belt to give you.
If you have the belt off and available use a cloth ruler to measure it around that you can buy at CVS in the sewing section.. If your belt shredder on you and you need to find a sized replacement instead of going by the make and model of your vehicle for some reason then run a string around the routing of the entire serpentine belt system as the belt would go around and mark the string once you've made an entire route. Then lay out the string and measure it with some kind of ruler.
Take the old one and compare them