I’d be fully satisfied with a 20x30 room that is fully dedicated for audio with no openings on any side. With most medium-sized speakers and monitors, a 16x24 room in the same design would make me quite happy. The floors would need to have concrete under as suspended floors resonate. The ceiling slant is quite important too. No angle causes the most first reflections. A 30-45 degree angle is too steep and increases cubic volume considerably, making it harder to fill, so a 10-15 degree angle would be my choice.
My room is 19x24 now, but on suspended floors, a fairly aggressive slanted ceiling, and an opening to the mid-to-back right.