That's gonna be tough. The best way with a non-standard horn would always be to take it in and try it out in various cases. Most euphonium "cases" are gig bags, unless you want to go for an MTS or something similar. However, doing that without knowing if it fits is always a risk, and the shipping is expensive.
It does seem to be "about" the same size as a regular euphonium/baritone. U.S. instrument makers didn't really make that type of distinction in many cases. It looks to be conical bore, which would make it a euphonium.
What's your budget for a case?
Sterling / Perantucci 1065HGS Euphonium, 1952 B&H Imperial Eb Tuba, and a bunch of trombones.