Since your Maui Jim sunglasses are always slipped down from your nose when you wear it, you should take notice of its nose pad position. You should use the screwdriver to adjust the nose pad which will be loose. You just adjust the direction of the nose pad for a little movement. Then you wear it to feel whether it is suitable.
You can adjust sunglasses by adjusting the nose pads on your sunglasses, to replace them on your nose. If it often slip down from your nose, you can tighten the nose pads to bring the sunglasses further up on your nose. However, you should not adjust your sunglasses frequently, as this can wear the frame material and make it easyto breakage.
As a matter of fact, indeed, there exist some glasses that are adjustable to different head sizes. However, if the Maui Jim sunglasses you have chosen is not able to adjust to your head size, there are no effective ways to tackle this problem. In the meantime, you might as well to roll up some handkerchief to the side of the frame. If this does not help, you should ask for replacement.