Horizontal Limit?

If teams choose to expand after the match has begun, is there a horizontal limit that the robot must meet?

No greater than 36"

that is 36 wide by 36 long
there is no height restriction after the start of the match

No, it is actually a point to point measurement per this Q&A. There is a difference, as a robot with a 40" horizontal arm can diagonally fit in a 36" x 36" square. So the rule is that no 2 points on the same horizontal plane of your robot can be longer than 36". Also no height limit after the start of the match as you said.

Look at the rulebook. It says 36" diameter cylinder of unlimited height.

There were many clarifications on it in the official Q&A. It is just simply 36" in any direction.