Please be aware that sizing yourself may not give you an accurate fit.
To ensure you get an accurate fit, you can purchase a sizing kit in the shape that you plan to order to make sure the nails you get look as natural as they can! 
To view the sizing kits, go to the Sizing Collection
What you'll need:
Clear scotch tape
A pen/marker
A ruler
A piece of paper
To Measure Your Nails:
Place a piece of tape across your nail bed and mark the sides of your natural sides at the widest point of your nail. Place the tape on a piece of paper and measure how many millimeters your nail bed is. (The smaller lines within centimeters)
Extra Small: 3  6  5  7  9
Small: 2  5  4  6  8
Medium: 1  5  4  6  8
Large: 1  4  2  3  6
Extra Large: 0  4  3  5  7
Sizes in MM (on centimeter side of ruler): 
**some sizes may be similar or close in measurement. Keep in mind, the larger the tip number, the smaller the nail size**
0- 13mm
1- 12mm
2- 11.1mm
3- 10.6mm
4- 10.2mm
5- 9.8mm
6- 9.4mm
7- 8.5mm
8- 7.8mm
9- 7mm