I'm also working on the TKS at the minute. It is certainly the smallest 1/35 AFV model I've built.
The French UE tankette (also made by RPM/Mirage) is nearly as small but is slightly longer.
As far as non-armoured vehicles go (not counting motorcycles) the Kettenkrad must be one of the smallest. (Though
it's actually slightly longer than the TKS and UE)
There's also a post-war French Jeep-type vehicle which is very small - can't remember the name but I saw it
in the Cobbaton museum, I think it was originally intended to be dropped by parachute - if there is a 1/35 kit
available for this, it might count as the smallest unarmoured military vehicle model!