There is not a standard set of dimensions for XL-ATX motherboards. XL-ATX motherboards are just any motherboard that is taller that than the standard ATX motherboard height of 305mm(12in). So it's harder to say that a case has "official" support for XL-ATX motherboards.

However, the Define 7 XL and Meshify 2 XL will support any XL-ATX motherboards up to 350mm in height.