To determine the size C11 or C12 for Crocs, you need to measure the length of your foot and compare it with the Crocs size chart. Size C11 usually corresponds to a US size 11, which corresponds to a foot length of around 27.2 cm. Size C12 corresponds to a US size 12, which equates to a foot length of around 28.1 cm. As the exact foot length for size 29 is not specified, I recommend consulting the official Crocs size chart to determine the right size.