The number sizes are supposed to use the circumference of your knuckles, sizes are as follows: 15cm = 6, 17cm = 6.5, 18cm = 7, 19cm = 7.5, 20cm = 8.
However, I don't always find this to work: I'm supposed to be a 7, usually get a 7.5 but had to get an 8 in my last leather gloves! So really it also depends very much on the material of the glove, the width of the cuff and the length of your fingers!