Start at the door. Before you read a single drum size, measure the narrowest point the machine must pass through: the door frame, a turn in the hall, or a landing on the stairs. Write it down with the depth of the room and the height to the underside of any shelf above the machine.
Clearance around the cabinet
Look at the side, rear, and top clearances the maker lists in the installation sheet. Those figures decide whether the machine can be pushed back far enough for the hoses to bend without kinking. Check where the hookups sit against the wall. A hose that bends sharply at the valve is one you will have to watch for years.
Water and drain
Note where the water valves sit and whether you can reach them with the machine in place. Then check the drain. Some closets use a standpipe, and the height of that standpipe sets how high the drain hose can rise before it needs a correction. Ask the installer or the owner of the house what the drain path is before you order anything.
The stack option
If the floor is tight, look at a stacked pair. A stack asks for height, a stacking kit or shelf that the maker approves, and access to the dryer's vent. Measure the top of the washer and the underside of any shelf above it. Compare the approved stacking method against the parts the maker supplies, not against what a sales floor says it can do.
What to check or ask
Check the width of the door opening at its narrowest point, the depth of the closet, the hookup position, the drain height, and whether the closet door swings clear when the lid is open. Ask whether a dryer will be added later, because that changes the air path and the shelf height. Ask the seller to confirm that the delivery route matches the same measurements.
The washer's typical service life and planning guidance are on the Clothes Washer page. Use that page for planning once the fit is settled.