The default size is 19.00 mm inner diameter but you can customize the size to your liking. Click personalize to choose your US ring size.(Size support: # 8 to # 13.) Please refer to a this page. And key type choose the command key or the ctrl key.
Conversion chart for ring sizes
US Ring Size Inside diameter 8 18.1mm 9 19.0mm 10 19.8mm 11 20.6mm 12 21.4mm 13 22.2mm