a. Once the Match begins, a Robot which is contacting the Expansion Zone may expand
vertically with no height limit. However, once fully outside of the Expansion Zone (i.e. no
longer contacting it), the Robot must return to a height limit of 18” (45.72 mm) tall.
b. Once the Match begins, Robots may expand, but no horizontal dimension can exceed 36”
(914.4 mm) at any point during the Match.
I don't see anything in the rules specifically against that, and the only specification for expanding over 18" is to be contacting the expansion zone with any part of the robot so I feel like it should be OK