Stored footprint is the real constraint

You will use the machine for perhaps an hour a day. For the other twenty-three it needs to not dominate the room. That makes stored dimensions, not set-up dimensions, the number to plan around.

A full-length reformer that stands on end parks in about 2 ft × 2 ft against roughly 8 ft of wall. A folding one parks in about 2 ft × 4 ft flat. Both are apartment-viable; a fixed 8 ft frame usually is not.

Noise and neighbours

In an apartment this is a genuine constraint, not a nicety. Carriage noise transmits through floors, and jumping work transmits a great deal. A dense mat under the frame helps substantially; avoiding jumpboard work in a shared building helps more.

Quality of bearings matters more here than anywhere. A machine that is quiet in a showroom is quiet through a floor.

The chair alternative

If a reformer genuinely will not fit, a Pilates chair is the strongest alternative. It occupies about 2 ft square permanently, needs roughly 5 ft × 5 ft of working area, and is a demanding piece of apparatus rather than a compromise.

Paired with a mat, a chair covers more of the repertoire than most people expect — and it never needs to be folded away.