You should measure the length of your feet first - Lems has provided instruction video and guide to measure your feet correctly. You can find the complete sizing guide here.
After you have the measurement refer to "Foot length cm" against the longer foot. Then pick the matching US size. All US sizes are Mens US for technical reasons. Womens US size is always size + 1.5 if you need to know it.
Recommendation is to leave 1-1.3 cm space for movement (sizing charts account for this). If you need additional room in the shoe, you can remove the insole for extra volume.