That's a very good question, to get your ideal diaper size you need to know your waist/hip measurement, it's usually done with a soft measuring tape used in sewing. There are many tutorials online on how to measure your waistline but after you get a number you'll want to check in the package or if you're ordering online in the manufacturer website a fit range usually located in the product description.
For example a Medium size adult diaper fits waists from 33 to 44 inches or a Small one fits waists from 21 to 29 inches, if your waist measurement falls in between of that specified range you should be good, but if it's above or below that number it's probable that it won't fit or be uncomfortable to wear.
Measure your belt line, comfortably, not tight. Add 4-6 inches. You may have to adjust the amount you add depending on how much wider your hips are compared to your waist. The big problem isn't the upper tapes, it's the lower tapes. No only do they have to go around the wider part of your waist (your hips) but the front panel also has to wrap in, around, and back out, taking the diagonal route, and this will pull the front lower wing in several inches more.
Opinions will vary, but I believe it's better to have a diaper that's too big, than too small. I want my diapers to be comfortable, not break tapes, and not bruise my hips. My diapers are for my personal enjoyment, and although the thought of a tight diaper is a bit enticing, the reality is usually much different.