[Chris is me]

Some of this has been said before, sorry.

I don't think any guess that doesn't account for the length of the box is correct. There is no reason to put everything in a 5 foot long box just to mess with people. It's so much more of a pain to ship, move around, and bring home an odd sized box than a more normal sized box; they wouldn't make the box long for no reason. Though the baseball theory has some merit.

Fabric often comes in standard widths of 54 inches. A possibility is that the box contains a mandatory - and new - bumper fabric material. This would address the subjectivity in the rules of what bumper materials are allowed and eliminate the advantages "engineered" bumpers give. The odd thing here is that the box weighs more than I would expect rolls of fabric to weigh. I wouldn't be surprised if they found a really tough and slippery fabric, engineered to help avoid T-bones, and maybe that weighs more than normal.

I don't think it's an odd, rolled up floor / driving material (like Regolith) - it's pretty hard to tightly roll somewhat rigid coverings into a 3" box. Perhaps it's a vision target, but that seems like a stretch; it hasn't been done in the past and they have moved toward retro-reflective tape anyway.

5 feet is the robot's max height from many years. Five feet of stock of a mandatory component or piece of extrusion is plausible. Perhaps it's "like" minibots, but robot side.