I’ve found
https://www.automobiledimension.com/car-comparison.php
pretty invaluable regarding car sizes. Not only can you compare up to 3 cars alongside each other but it will show you other cars with similar dimensions.
https://www.ridc.org.uk/features-reviews/out-and-about/car-search
is very good too for more detailed dimensions like door and sill heights of many cars.
There are tools to be used beyond Motability.