To find your size, measure the width of your nail bed at its widest point using a soft measuring tape.
Alternatively, you can place sticky tape across your nail and mark the edges of your nail bed with a pen, then place the sticky tape on a piece of paper and use a ruler to get the millimeters!
Small
✦ Thumb - 2 (16mm)
✦ Pointer - 6 (12mm)
✦ Middle - 5 (13mm)
✦ Ring - 6 (12mm)
✦ Pinky - 9 (9mm)
Medium
✦ Thumb - 1 (17mm)
✦ Pointer - 5 (13mm)
✦ Middle - 4 (14mm)
✦ Ring - 5 (13mm)
✦ Pinky - 8 (10mm)
Large
✦ Thumb - 0 (18mm)
✦ Pointer - 4 (14mm)
✦ Middle - 3 (15mm)
✦ Ring - 4 (14mm)
✦ Pinky - 7 (11mm)