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.