Bed lengths for reference...
54" Rivian R1T
54" Ford Maverick
64" Honda Ridgeline
66" F150 Lightning
I own a Ridgeline. The bed is fine for chucking some camping gear, but it's nowhere near long enough for lumber, sheetrock, etc. Mountain bikes don't fit upright, even with the front wheel off.
The Rivian is 17" longer overall than a Maverick and 7" longer than the Honda. It's bed is really small - fine for casual use, but hardly suitable for "working truck stuff". And that's not the market Rivian is targeting, so that's fine.