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A good question that! Too tight and I get weals and soreness. Too loose and gaps appear, which means either a leak or my nappy pokes out through a gap and wicks into your outer clothes. A key part of this, in addition, is making sure the pants are big enough so that gaps don't appear as you're moving around, due to the bulk of the nappy. So the pants need to be quite a bit bigger than the nappy, which leads to the problem that pants that big may well have elastic that's not tight enough for the size of your thighs. I find that most plastic pants aren't cut right to cope with bulky nappies and slim thighs.
In addition, there are pants around where the elastic itself wicks wetness through to your outer clothes. These are just no good for cloth nappies. You need either enclosed elastic (which I don't actually like due to the difficulty of keeping the elastic clean), or elastic that doesn't wick.
Having said all that, I tried a lot of pants before finding the ones that work well for me, and you'll probably have to do the same. For what It's worth, I wear these, made by Gary & sold by LLMedico. They're just the job for my nappies and body shape, & if you're lucky they may work for you too.