Clincher sizing standards use the diameter of the bead and bead seat along with rules for where to measure and the dimensional profile of the bead and bead seat.
Tubulars don't have beads or bead seats. They have the contact area between the rim bed and the base tape, and that's it. One can imagine a standard based on the diameter of the the imaginary circles running along the middle of these surfaces, but does that exist in anything other than an informal way?
Tubulars are stretched to fit. If tubular compatibility is in fact based on informal or de facto standards, would changing to a formal standard based on the diameter of the rim bed and the ID of the base tape of the inflated tire eliminate the need for pre-stretching?