I believe the rules say the robot has to be less than 15x15x15, not less than or equal to 15x15x15.
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.
this is for vrc, but I assume the smaller keyword carries over to vexu.
so if we assume that vex parts are perfect, then a robot exactly 15" long would not pass inspection.
however, vex parts are not perfect, and just from the wiggle room in holes, you could theoretically build a base using 30 hole c channels that is smaller than 15" x 15". I wouldn’t dare push it this close though because sizing tools are not super reliable and at the end of the day, sizing tools are what inspectors are going to look at. even if your robot technically is 14.99", if it won’t pass through the sizing tool you’re going to have a hard time convincing the inspector to let you pass.
although I’m not sure how robots are sized for vexu. are there sizing tools?
in that case you have a better chance at passing. still wouldn’t risk it, but tape measures are significantly more reliable that sizing tools, so your chance of passing is greatly improved.
VUR-1 is the rule for VEX-U sizes, found in appendix D of the game manual, and like the VRC manual, the robot must be smaller than 15".
If you want to push the size limits, you should use versachassis and shoot for 14-7/8. The inspectors will appreciate not having to deal with problem robots/teams. Most U-Teams build to 14-1/2 max if using c-channel.