-Place tape over every nail and mark your nail walls then place that tape on a ruler and measure! (Like shown in the picture!)
OR
-Using a small soft measuring tape (sold at your local craft store, Michaels, Walmart, etc) measure the widest part of your nail bed on each finger starting with your thumb. *only nail bed, not skin around nail* Be sure to measure in MM.
(it's always good to be a half size bigger if need be than smaller, if too small, your nail could pop off easily by not being flat on the nail bed*)
Pre-Set Sizes: Most people's nail measurements fit these pre-set sizes!