A refrigerator is sized to the way food moves through a house. Start with your habits, then measure the spot, then compare models. Working in that order keeps you from buying room you will not use or buying a box that does not fit the doorway.
Start with how you shop
Look at how often you buy food and how much you carry home at one time. A household that shops every few days needs less stored room than a household that buys in bulk once a week. Note what is always in the refrigerator: milk, condiments, leftovers, drinks, and prepared meals.
Count the cooking and the guests
A household that cooks large batches or hosts often needs more shelf area and more freezer room. Note how often you cook at home and how many containers you store at a time. Note whether you keep large platters, tall bottles, or bulk packs that take up unusual space.
Check the space before the model
Measure the opening for width, depth, and height. Allow for the clearance the manufacturer requires at the back and sides for ventilation, and for the handles and door swing. Check whether the door has to open beside a wall or a counter. Note where the power outlet sits and whether it can be reached without moving the unit.
Choose the layout
Decide how much freezer room you need, and whether a drawer, a bottom freezer, a top freezer, or a side-by-side layout suits the way you reach for things. Each layout trades door space against shelf space. Ask the seller to show interior dimensions and not only the outside measurements.
What to compare
Compare the interior capacity against your storage pattern. Compare the usable shelf area, the depth of the door bins, the door swing, the freezer share, and the placement clearances. Then check how the shelves adjust and how easily the drawers slide out. Once you have a shortlist, the page on typical service life for the six major appliances gives you a planning date to set beside the purchase.