Ok here is a slightly tangential answer to my own post.
Wacom Intuos Small
It’s a bit on the large side at 20cm x 16cm but it has bluetooth and supports Windows, Mac and ChromeOS, and can be used as a pointing device with advanced gestures.
That’s 4cm wider and deeper than the magic trackpad with probably the same active area (the outside edges of the Intuos aren’t touch sensitive) but it is almost half the price of the magic trackpad and fully supported on other operating systems.