how big horrizontally can a rocket stove be?

How big can the horizontal portion of rocket stove be? I am asking because I am wondering if it can be built in a way to heat the entire floor of a mud/cob building.

If you are talking about the burn tunel, not long enough, for sure. May be a foot or two at max. If you're talking about the flue, Some went up to 40 horizontal feet with an eight system. Remember tho, to insulate your cob floor from the surounding ground, as it will act as a heatsink otherwise. Sides and bottom insulation, as well, chich can wistand the weight. Over here, for concrete slabs, we use styrofoam.