I have an American Eskimo dog, which is built a lot like an elkhound but smaller (35 pounds) and fluffier; she has a very thick ruff so it probably works out to corgi size or slightly larger (maybe Cardigan-size instead of Pembroke?). She doesn't normally wear bandannas since she's already "overdressed" with her own fur but the last time I had her professionally bathed she came back in one that was probably about 28 inches on the hypotenuse, tied loosely.

The same size would probably fit my friend's Cardigan/heeler/pit bull/?? mix, who is about the same height as my dog but much longer and thicker (almost 50 pounds), with a bit of a bull neck but no ruff.