I've also looked a bit at the kenwood steering wheel remote and hacking that apart... how big is it? Small enough to possibly fit into a highly modified knob?

Approx. 42x31x13 mm (divide by 25.4 for inches) 13 mm is "thickness" w/o the buttons. Measurements w/o the LED which is in a separate enclosure at the end of a 20 cm (divide by 2.54 for inches) wire. If you can place the battery elsewhere you can, at least, halve the thickness.

/Michael