Sectionals split into two build types that shop very differently. A fixed-frame sectional is built as one continuous L or U shape and, once assembled, cannot be taken apart again — if it doesn't fit through your door in one piece, it doesn't fit. A modular sectional ships as individual sections, usually 60 to 120 pounds each, that connect on-site with metal clips or hook-and-loop straps; each piece can be carried in separately and the whole thing can be reconfigured later or split when you move. If your building has narrow stairwells or a tight elevator, modular is worth asking about by name before you fall for a fixed-frame floor sample.
Underneath the upholstery, look for a kiln-dried hardwood frame — kiln drying brings the wood's moisture content down to roughly 6 to 8 percent, which keeps joints from loosening as the frame acclimates to your home over its first year. Corners should be glued, screwed and doweled or blocked, not just stapled. The seat platform is usually either sinuous wire ("no-sag") springs clipped front-to-back across the frame, or the more labor-intensive eight-way hand-tied coil system found in higher-end pieces; either can be good, but a salesperson who can't tell you which one is under the cushions probably hasn't been trained on the product.
A standard sectional runs 110 to 140 inches along its longest side, with a chaise depth of 62 to 70 inches. Measure your room's longest unbroken wall, then subtract 24 to 30 inches for a walking path around the piece. Before you order, measure the diagonal of every doorway and stairwell landing between the truck and the room — a good store will ask for these numbers before scheduling delivery, not after the truck is already there.
A retailer worth the drive will let you sit on an actual frame sample, quote a real fabric or leather grade rather than "call for pricing," and know whether a given sectional can be special-ordered in a different configuration. That combination — construction knowledge plus configuration flexibility — is a better signal of a good sectional shop than square footage alone.