The Japanese wear geta with the heel hanging 2-3cm off the back, to centre gravity of the body, though this is less essential with some modern style geta, which have a heel and don't have the narrow teeth under the sole. The type with two teeth are worn this way (see diagram above this pair of geta’s photos). Measure your foot when standing (or squatting), with your weight pressing down on it, then compare with sole length of geta. Note that a, for example, 27cm sole is suitable for a 30cm foot when worn the traditional way with the heel slightly hanging off the back of the sole.