It depends on how you want them to fit, but I think at 36" you are getting the top tapes very close to each other in the front. I'd suggest medium for ABU brands for you. I measure for about a 42" around-the-waist fit on my diapers, and ABU are spaced about 8" apart in the front for size Large. If that continues downward for you, you'd only have 2" or so between the top tapes, and that's probably going to fit pretty poorly.
That being said, "get samples before ordering in quantity". Who knows, you may like an oversize diaper? If you want to get more than just samples to start with, get a bag of medium and a bag of large, preferably of different styles. (space, paws, preschool, and sdk would be on my list of options) ABU has some nice sampler arrangements in their store. Tossing out a specific suggestion, get two premium samplers so you get one of each of the four options, two of them medium and the other two in large. You'll get at least four diaper to try, at least two will fit good, and you'll know exactly what you want to get next, for type and size.