When I try to save a PDF with the page size 446mm x 206mm it comes out at 446mm x 206.1mm. And if I set the height to 205.8mm it comes out at 205.7mm.

How can I make it exactly 446x206?

This is caused by page sizes getting rounded to the nearest point (1/72"), and is fixed in Prince latest builds.

I tried updating to the latest build, that fixes the height issue, but instead applies the issue to the width, so it comes out at 446.1mm x 206mm instead...

That's weird. When I try it here the size is displayed correctly. Checking the PDF file the /Page object includes this line:

/MediaBox [0 0 1264.2520 583.9370]

1264.2520pt == 446.000011111mm, which hopefully is close enough!

Cheers for that, using points seem to do the trick