If you're looking to have a cargo box installed on the roof of your Mini Cooper, here are some photos to get some perspective on the size.  The box in this photo is a Thule Atlantis 1600 - Black.  It's installed on the OEM roof bars that the customer had installed at her Mini dealer.

Note: The Thule Atlantis 1600 is no longer available.  There are a few new Thule boxes that will be similar to the size of the Atlantis 1600.  The Thule 615 Pulse XL (16 cubic ft.), the Thule 625 Force XL (17 cubic ft.),  the Thule 635 Sonic XL (17 cubic ft.).

Since the roof-line of the Mini is so small, we didn't really want to go with a much larger Thule Cargo Box.

Thule 686BXT Atlantis 1600 Cargo Box
The 686BXT attaches directly to the OEM Mini Crossbars with the included QuickGrip Mounting Hardware
Thule Roof Cargo Box Atlantis 1600 Black
The placement of the Cargo Box allows the vehicle's rear door to open without interference, even with a small spoiler
The short roofline of the Mini dictates the size of the roof box that will fit properly
MIni Cooper Cargo Box Install
The Thule Atlantis 1600 complements the roof of the BMW Mini Cooper.