What rootstock offers the most compact size outside of Fly Dragon?

I'm no expert, but I'll contribute my two cents.

Four Winds sells a lot of trees that are labelled "dwarf". The ones I have asked about are budded onto ordinary P. trifoliata. I have one of their Robertson navel oranges, which I espaliered against my chimney. It is about 18 feet tall, but only ~3 feet deep and about 6-8 feet wide. "Dwarfish" I would say.

An article describing a test of various rootstocks (comparing the size of the root systems) noted in passing that using yuzu as a rootstock produced the smallest scion growth of the stocks tested.

Sorry, that's the extent of my knowledge!