The normal Cooper.

The 4 door Cooper is the same width as the 2 door, 1cm taller, and 16cm longer, based on a 2013-2018 model since that’s what this tool has available:

https://www.carsized.com/en/cars/compare/mini-cooper-2013-3-door-hatchback-vs-mini-cooper-2013-5-door-hatchback/

Compare a 2 door Cooper vs 2 door Golf and it’s barely different:

https://www.carsized.com/en/cars/compare/mini-cooper-2013-3-door-hatchback-vs-volkswagen-golf-2012-3-door-hatchback

Or 2 door Cooper vs Smart Fortwo:

https://www.carsized.com/en/cars/compare/smart-fortwo-2014-3-door-hatchback-vs-mini-cooper-2013-3-door-hatchback/

The Smart is clearly a smaller size. The Cooper and the Golf are basically the same. There’s definitely a lot more cross shopping of Coopers and Golfs than Coopers and Smarts.

A modern Mini Cooper is nothing like a Smart car. It’s comparable in size to a Golf, or a Corolla hatch.