I just checked, apparently we have a ~16 gallon can (15 7/8ths... Rubbermaid 3541, "Slim Jim" not cheap for plastic) and use 33gal bags. I doesn't seem that much bigger but an overfilled bag will still have room in it when removed (it's useful for last-minute additions on garbage day).

I don't know if you need to go this far, but maybe it is why they still fit the can properly with the not-expected-fit orientation like I described to prevent overfilling. 30gal might work, I guess it seems there isn't much choice here though (otherwise I'd say try 5-10gal higher).