I only expect the Y to be slightly smaller than the X (internal volume), and almost as long (reducing length increases, not decreases, aero drag). ~20% less cargo space than the X = Rav4 sized (Rav4 = highest selling SUV in the US). In varying markets / generations, Rav4 has been offered in 7-seat configs. As I expect Y to be longer than the Rav4 (but more tapered in the rear), it makes a 7-seat config easier.
Hard to say what Tesla will do; time will tell. But 7-seat configs are both A) possible, and B) have market appeal. Remember, Tesla doesn't sell dozens of different vehicle models, each to its own niche; it needs to fill in as many niches with each model as it can if it wants to move 500k 3 / 700k Y per year.
(The easiest 7-seat config, IMHO, would be rear-facing seats in the back. That way you maximize hip/shoulder room despite the rear taper)