I got excited to see the Casio SA-76, which almost fit the description, but it doesn't have a 1/4 inch jack output.
I do not care about timbre or sound quality, because I want to run the keyboard through a vocoder while having access to the voices on my other "real" keyboard.
By smallest, I mean fewer than 49 keys. Smaller means lighter, and it's more aesthetic to have a tiny keyboard sitting on top of my other keyboard.
EDIT: Just to clarify, I don't mean a MIDI controller, but a keyboard that outputs its own tones.