Wrap a tape measure around your hand using the inches side around the broadest part of your hand as per the photo below and read off the size in inches, that is the size of your hand. 

7 inches = a size 7 hand 
7½ inches = a size 7½ hand
8 inches = a size 8 hand 
8½ inches = a size 8½ hand
9 inches = a size 9 hand 
9½ inches = a size 9½ hand
10 inches = a size 10 hand 

Here is a guide to what flute you will be able to play using your hand size,
      - size 7 hand G# and G
      - size 7½ - 8 hand G#, G and F#
      - size 8½ - 9 hand G#, G, F# and F
      - size 9½ - 10 hand G#, G, F#, F, E and D#