For covers you are best to find a product where you fall close to the middle of the size range listed. You do not want it to be too loose or to tight. If you are wearing the pant over something bulky, we recommend taking your measurements to include the bulky item.
Manufacturers list the absolute minimum and maximum sizes the garment will fit. These measurements do not take into account body shape, weight or height.
If a brief lists an upper range of 44", this measurement is with the tapes JUST touching the edges of the front of the brief. If you want 2" of overlap on each side for example you will need to deduct 8" from the maximum to asses a realistic fit. So if you have a 36" waist it may or may not be a good fit depending on other factors. It is always best to order a sample or bag before buying a large quantity of a new brand or style.
These are more forgiving in size, as the fasteners are much more durable than disposables. You will be most comfortable in products where you do not fall close to the upper or lower range of the size listed.
Rearz brand onesies are quite stretchy. You should measure shoulder to bottom of crotch over top of your briefs. The length for support is most important. The chest has 20% stretch, so that is the 2nd most important measurement.