Is it required to use the official VEX sizing tool for video-inspection in a pre-recorded skills competition? Or can we use our own, home-built, 18"x18" sizer? Would we have to show on the video that the sizing tool is indeed 18x18?
yes, size the sizing tool
Any question that starts “is it required/is it legal/are we allowed” must be answered by the game manual, which is the only official source, along with the official Q&A, for the rules.
In this case, we refer to R5 of the game manual (and the “red box” therein), where it give two choices for measuring: either a sizing box or one of the two VEX sizing tools. The “sizing box” is indeed a “home-built” tool, and for remote-competition, it might be good to show measurements of hour home-built sizing box.