Correct fit matters more than the belt design itself. Each Fosta size corresponds to a range in the brand's size chart, so measure yourself according to their instructions—usually around the waist or hip where the belt will sit—before choosing. Do not rely on your usual clothing size.
When you put the belt on, it should feel snug but not tight enough to restrict movement or breathing. You should be able to slide two fingers flat between the belt and your body. The upper edge should sit below the ribs and the lower edge above the hip bones, with the rigid ribs centered over the lumbar area. If the belt slides, rolls, digs in, or creates pressure points, try readjusting the compression straps before considering a different size. If it still does not fit, compare your measurements again with the size chart.
Remember that the mouldable ribs can be shaped to your back, but they cannot compensate for a belt that is too short or too loose. If you are between sizes, the manufacturer's chart is the decisive guide; never buy a larger size expecting the straps to fix a poor fit.