Measure the run, then the depth
Soffit covers the underside of the eave overhang. Walk the house with a tape and total every eave run — front, back, both sides, plus any porch or bay overhangs — then multiply by the eave depth (usually 16–24 inches from wall to fascia). Enter the run as length and the depth as width above.
Solid or vented — the 1:150 rule
Attics need 1 sq ft of net free ventilation per 150 sq ft of attic floor, and the soffit supplies roughly half of that at the intake. In practice that means every other panel is vented on most houses — order a vented/solid mix rather than all-solid, or the attic will breathe only through the ridge.
Aluminum or vinyl
Both ship as 12-ft strips, 12 or 16 inches wide. Aluminum costs more and dents less; vinyl is cheaper and cuts with a utility knife. Either way, nail the J-channel at the wall and fascia first — the panels themselves take zero nails, they float in the channels so the metal can move with the seasons.