The Beetle rules are the same as the rules for all other weight classes under "Health and Safety".

If there is no mention of a rule, then it doesnt apply unless the EO makes it an additional. For example if there is no mention of a size limit, it is because there is none. So long as the robot fits through the arena door it can enter a fight.

The only real rule difference beetles have compared to other weight classes is allowing 2 robots per driver.