While the sizing tool is a useful tool to have at events, the Game Manual is always the official and final ruling on what should be considered legal.
Rules R4 and G4 are quoted as follows:
<G4> Robots begin the Match in the starting volume. At the beginning of a Match, each Robot must be smaller than a volume of 18” (457.2 mm) long by 18” (457.2 mm) wide by 18” (457.2 mm) tall. Using Field Elements, such as the field perimeter wall, to maintain starting size is only acceptable if the Robot would still satisfy the constraints of <R4> and pass inspection without the Field Element. Robots in violation of this limit will be removed from the field prior to the start of the Match, at the Head Referee’s discretion.
<R4> Robots must fit in a sizing box. At the beginning of any Match, Robots must be smaller than 18” (457.2 mm) long by 18” (457.2 mm) wide by 18” (457.2 mm) tall.
a. Robots may expand beyond their starting size constraints after the start of a Match.
b. Any restraints used to maintain starting size (i.e. zip ties, rubber bands, etc.) MUST remain attached to the Robot for the duration of the Match.
R4 also contains a "red box" as follows. "Red boxes" are used throughout the manual to provide clarifying statements in situations where the strict verbiage of an official rule may require some additional context.
Robots will be measured by either being placed in a “sizing box” with interior dimensions matching the above size constraints or by using the VEX Robotics Competition Robot Sizing Tool while the Robot is placed on a flat surface. A Robot may not touch the box walls or ceiling or the Robot Sizing Tool sides when being measured. There are two VEX Robotics Competition Robot Sizing Tools that may be used: https://www.vexrobotics.com/276-2086.html and https://www.vexrobotics.com/276-5942.html
Neither G4 nor R4 are written to explicitly state that "Robots must fit in the sizing tool"; instead, they state that a Robot must be 18" x 18" x 18", and the red box clarifies that it is at the EP's discretion what tool they wish to use to test this compliance.
Teams should design their Robots with this possibility in mind; that is to say, an 18" x 18" x 18" Robot should pass inspection regardless if the event is using an old tool, a new tool, a tool that is slightly bent, a tool that was manufactured out of tolerance, or even a home-made "sizing box".
