If you have a flexible measuring tape, you can simply use it to measure your nails from left to right at the widest point of your nail and note down the measurements in millimeters for all 10 nails.
Without a measuring tape:
To measure your nails without a measuring tape, you can simply use tape. Stick it over your nails and use a pen to mark both sides where the skin meets the nail. Remove the tape and stick it flat onto a smooth surface. Now simply measure the distance in millimeters with a ruler and note it down.
It's best to repeat this for each of your 10 nails, as it also happens that one has different nail sizes on the right and left.
If you are unsure about measuring, you can also order a sizing kit here. Returns or exchanges due to incorrect sizes are unfortunately not possible, as these are handmade products customized for each customer.