You have some high expectations Daniel!
I know of no function-only decoders (any physical size) which would include a keep-alive circuit. They also require current limiting resistors.
MY recommendation is
ZIMO681It has 6 function outputs and it is quite small .47 x .33 x .086 inches (12 x 8.5 x 2.2 mm).
ESU also has a nice function-only decoder,
ESU 54620, but it is a quite a bit larger. I have not used that one, but I'm sure it works well.
You can always go with the Digitrax or TCS decoders, but I was not impressed with their lighting effects (like alternating ditch lights). Digital still has not figure out how to properly drive LEDs (the lighting effects). Even in the newest decoders like DZ126.