On the site's size chart you'll find EU, UK and US sizes alongside foot length in millimeters and inches. M stands for men's and W for women's sizes, so a label like M4 | W6 means one shoe that fits a men's 4 or a women's 6.
Place your foot on paper, measure from heel to longest toe, and compare the result with the chart. Falling between two sizes? Then the guide says go for the bigger one. Crocs already have a roomy fit, yet the guide still advises wide feet to size up, and fit can differ between models, so check the product description before ordering.
Source: Crocs Gulf Saudi Arabia: size guide