To work out your size for riding gloves you need a tape measure with inches.
Run the tape measure around your palm, just under the fingers, and the measurement will be your size in gloves.
Generally riding gloves are available in sizes 6-11. The most common ladies sizes are 7.5, 8, 8.5.
If the gloves are sized XS-XL, then convert them like this: XS=6, S=7, M=8, L=9, XL=10