"real" Baltic Birch comes in 5' x 5' squares. AFAIK any other size is imitation and not the real thing. I've also never seen a sheet in other sizes that was worth the asking price because often the imitation stuff is poorly made but just as expensive as the real thing. The real stuff is also metric thickness, so half inch BB is actually 12mm and quarter inch is 6mm.
I believe the smaller pieces that Woodcraft sells are cut from real BB as IIRC they offer sizes that would make sense, like 30x30 and 30x60.