I have about a 40" waist, but with a little bit of a gut. I normally size my daytime diapers for around 40" but my overnight for more like 42 so they fit loose and comfortable.
I can't really fit into any mediums (the lower tapes don't make it to the front panel usually, but the top tapes usually can reach the landing zone) so I wear large. A few of the large fit larger than others though. For reference, on most of the diapers I wear, the pull tabs on the tapes end up right around where the two "factory folds" are on the front. A few diapers tape closer in, a few tape farther out. (and a couple tape waaay farther out, like qtty rawrz) So I suppose I can say that I'm not "between sizes", and so I don't have to go with different sizes depending on the diaper.