I am looking at this page and not sure why are any sizes larger than 15x16 are on for sale? Doesn’t competition prohibits for any robot to be larger than 18x18? https://www.vexrobotics.com/vexedr/products/accessories/structure/chassis-kits.html
Each hole represents 0.5"
I’m not sure if VEX does actually make a chassis larger than 18", but keep in mind that not VEX doesn’t make parts just for the competition.
The “35 x 35” chassis measures just under 18" x 18", so is the largest legal chassis for VRC, and there are no standard VEX parts larger than “35-holes” (remember a chassis kit is just a convenient collection of stock VEX parts, you don’t have to specifically purchase or use on on your robot). But if you consider some other applications, longer parts (which don’t exist) would be handy: you can go “49-holes” on a diagonal “arm” and stay withing the 18" cube (even longer if you go across opposite corners of the cube), and in VEX-U, the starting size is 24" cube, so U-teams are often splicing parts together for longer lengths.