How to Properly Size
See the Best Fit Size Guide.
If in doubt, measure foot to compare to the max foot length for each sandal size (as indicated on the Best Fit Size Guide).
If at the max foot length, you can download & print out the two size sheets. Then stand on each of the images and make your decision.
When printing please be sure scaling is set to "None" and printing options are NOT set to "Fit to Page."