When looking for good fitting plastic pants, you need to consider several size measurements, or factors. First, a pant that is a bit over sized, or larger then what you think would be right for you, needs to be considered. They need to fit over your biggest, thickest diaper that you will wear, and then some. They need to be a bit balloon like over the diaper. Next, you need to have your waist size dialed in good. The waist needs to be snug, so the pants won't slide down so easily. Not to tight, as to be uncomfortable, and not to loose, as to be able to slide with the weight of the pants and them pulling, when you move. You also need them to be snug enough, so you don't have leakage at the waist.
They should be somewhere in the middle, of the size range of the pants. For example, If you are a size 40, you would need the pant size to be from say, a size 34,36-46,48, or there about. You want to fall into the mid range, or maybe slightly to the tighter side. The same would be true for the leg holes. But you may want to go a bit more tighter here. This is were you would leak most, so going a bit tighter is better. This is all contingent on your own comfort level too. Some don't like tight leg elastics, they can leave marks when worn for long periods. If you change right after you wet, you can go with a more loose fit leg too. But for like an over night pant, they need to be tighter in the legs, and the waist.
Custom size pants are the way to go for lots of people. You can have them made to fit you best this way. They can then be made big, and balloony in the body, to give you the most room over your diapers, and you can have the leg and waist bands fit in your best mid zone size.
Sometimes you need to experiment, with what will work best for you. Buy more then one pair, but in different sizes, so you can see what works best for you.
You can put lots of thought into your plastic pants, if you want them to be the best they can be for you, and your situation.
I think that's the short of it... Hope it helps with, those plastic pantie pains?