The smallest single-person motorized vehicle is a motorcycle. Leaving stability aside for a minute - one of the issues with electric motorcycles is that the batteries are too small to have range that most riders consider sufficient.

If you have a large enough battery to deliver sufficient range for an electric taxi (driving around most of the day, hopefully, if it is well-utilized), with 4 wheels for stability... I doubt that you could really get significantly smaller where the smaller size would make a big difference.