Rules wise, it would appear not to matter, but logistically, you might care. The 28" dimension is chosen to be narrower than a door way, and I suspect the 42" dimension is either based on the field gate opening and/or some 'standard' for how tight a corner needs to be turned in standard building/equipment/hallway/doorway scenario.
In other words, you may need to move your bot through a doorway, and if it is a 'wide' bot' with a 42" front face, you won't be able to roll it through a standard doorway, unless you put it on a cart with casters or wheels oriented 90 degrees to the bot's wheels. But if the bot is max height, you may not be able to roll the cart + bot through the doorway due to height reasons.
Worst case, you have to drag or slightly lift your bot to get it sideways through a standard doorway.
A year or two ago, a team built a circular bot with the max 112" perimeter-- this plus bumpers meant it would not fit through a standard door-- they were given special permission to take a different route back to the pits at 10K Lakes regional, so that they didn't have to pass through the women's restroom like the rest of the teams. (yes, the restroom was closed for normal use.)
I'm waiting for a bot this year to have a 28x42x78 Transport config with wheels oriented perpendicular to the diagonal, so that it needs a 50.5" doorway if they want to roll the bot through the doorway
