A garage door opener has to match the door it lifts and the space its rail and motor will occupy. The fit question comes before the model question, and the door is the first thing to measure.
Start with the door
Note the door style, whether it is a one-piece or sectional door, and whether it tilts or lifts on tracks. Note the door's weight if the maker's label or manual states it. Look at the tracks and springs, and note whether the door already needs repair, since a motor cannot correct a door that binds.
Measure the headroom
Headroom is the distance from the top of the door opening to the ceiling or the lowest obstruction above it. Measure it in inches at the center and at both sides. Short headroom can rule out a standard rail layout and push the choice toward a different mounting. Ask the installer which mounting layout the measurement supports before any unit is ordered.
Measure the ceiling run
The rail runs back from the door toward the rear of the garage. Measure how far the rail will extend and whether the ceiling framing can hold the mounting brackets at the right points. Mark the joist positions, and check for ductwork, lights, or storage racks in the path. A rail that cannot clear them has to be planned around before install day.
Check the power and the reach
Note where the outlet is and whether the cord will reach the motor without strain. Ask whether a dedicated outlet is needed. Keep the cord clear of the rail and the moving door.
Match the rating to the door
Ask which rating the proposed unit carries and which door weight that rating is meant for. Do not accept a rating offered without a matching description of the door. If the door is heavy, unusually sized, or needs spring work, that work belongs to a trained technician and should be scheduled before the opener goes in.
The opener guide turns an install year into a planning date and lists the typical service life for the unit. Read it once the year from the nameplate is known. Garage Door Opener: typical service life and when to plan