To establish your personal mouse size, measure the length from the crease of your wrist to the end of your longest finger:
Less than 17cm - Small (Not available left handed)
17cm - 19cm - Medium
19cm - 21cm - Large
More than 21cm - Extra Large (Not available left handed)
The Contour range is established as the definitive 'ergonomic' mouse offering a choice of sizes in both left and right-handed variants.