afaik it's the Oppo N1[1] running Cyanogen. I got to play with one at the Big Android BBQ and it was pretty sweet.

[1] http://en.oppo.com/products/n1/

A 5.9" screen seems huge - would it fit in an ordinary trouser pocket?

It fits in mine... but I also wear 36x38 jeans since I'm 6'9", so I'm not a typical use case. My current phone, a Note 3, fits with a lot of room to spare so I suppose it depends on the size of your jeans.

I can also use these phones one handed whereas I don't see most people doing that.