The rule defines the max expanded robot size seems to be not present in the current VEX Tower Takeover game manual. Does that mean that there is no such expanded size limitation as long as the robot meet the initial 18’‘x18’‘x18’’ starting size limit?
Yes. There is no size limit.
However, there is the obvious size limit of 12’x12’, which makes up the size of the field. (And exiting the field results in a penalty to some degree depending on severity.)