A mobility scooter does not know where it is going to live. You do. The first measurement that matters is not the showroom floor but the trunk, the back seat, or the garage corner that has to hold it on the days your mother is not riding it.
Local option: New Plan Care gives families in the St. Louis area in-home help so a parent can stay home: companionship, help with bathing and dressing, meals, and overnight care. Call (314) 405-0887.
Measure it folded
Ask for the folded length, width, and height, and the weight of the heaviest piece. Many scooters come apart into a base and a basket or seat, and those parts are what you actually lift. Write the numbers down and carry them to the car. Then load a box or bag of the same size into your trunk and see whether the scooter fits beside it with the hatch closing and nothing hanging out.
Measure your lift
Be honest about who loads it. If you are the only person who can lift the heaviest piece into the back of the car, that number decides the purchase. Ask the seller whether the piece can be separated into lighter parts, and whether you can practice the lift in the store parking lot before you take it home. A scooter that only one person can load is a one-person job, and that person is you.
Measure the route
Look at the path from the car to the front door and from the door to the room where the scooter will be kept. Count the steps, note the threshold height, and check the door width. Many older houses in St. Louis County have a step at the front door or a narrow hall. A scooter that stops at the threshold will be used less, and a scooter used less means the trip you were paying for becomes a trip you take yourself.
Where it parks and who charges it
Pick a spot near an outlet you can reach without an extension cord. Write down the charging routine and name the person who checks the charge. Name the person who moves the scooter if the garage needs the space. The sizing question is not finished until someone can answer where it sleeps and who keeps it going.
If the lifting or the driving is wearing you down, look at the Local help page for options in the area before you buy anything that depends on one person's back.