Sloping TTs have nothing to do with fit. The frame size numbers have always varied between brands. Some measured c-c, some c-t and the real oddballs like Trek and Fuji would measure to the top of the seat tube, even if it extend several cm above the TT. A 56cm Trek might fit like a 52cm in a brand that measures c-c.

If you really want to narrow down the relevant frame numbers, use reach and stack, but only a few brands list those dimensions. Even those can be misused.