Choose size 6 only if your waist measurement falls within the manufacturer’s size chart for this model. The source information specifies that size is selected by waist measurement, and this is important for an elastic lumbar belt: if it is too small, it may create uncomfortable pressure; if it is too large, it may shift, bunch or fail to provide even compression.
When putting it on, position the wide part so it covers the lumbar region and the lower part of the torso. Fasten it firmly enough that the fabric hugs the body and gives a stable feeling, but not so tightly that it restricts breathing, presses into the abdomen sharply or causes numbness. You should be able to stand, bend gently and sit without the belt rolling up or digging in.
Check the fit during the type of activity you plan to use it for, because a belt can feel different when lifting, driving or doing housework than it does while standing still. The LAUMA Extra model has a wide profile intended to cover the lumbar area without rolling when you bend or sit, but correct size and placement still matter. If the belt repeatedly rides up, folds, leaves strong marks or needs constant readjustment, recheck the waist measurement and compare it with the size chart.