Usually with the tailgate down you can fit 4x8 sheets of material
A least according to articles written in the past few months, they only claim a bed that's "longer than 6 feet", not 6.5 feet per se.
Oh, I'm sure there will be some technicality that they point at and say, see, it's 6.5" and then will sell a cybertruck bed extension next year for a cool $23k to make the space usable.