I am 72 inches tall and my wife is 70 inches tall and we both have large feet. To properly center our feet wee have to put our toes over the end of the board top. A larger board top would be most welcome.

No, you only tip over more easily from the board and can hurt you. I don't have any difficulties with that (76 inches ).

I agree about a larger balance board. I'm a big gal, and a bigger board would be helpful.

A large board is possible but the feet must by on the very edge or you get into big troubles.
However this larger board will also not so good by trainings you have to go sidewards off the board like Rhythmic Step.

The size of the new top would only have to be deeper not wider...we have 35 and 36" inseam so side stepping is not that difficult. besides if this was sold as a separate piece you could buy the one that fits your feet!

My son is 6'3" and wears a 13 shoe but he says the board is fine. I guess it depends on your preference.

13 in UK or US man size? There is a difference of one hole number in between.
My little grandson also has difficulties to remain on the board, but he is only 3 years and his shoes are safe not too big.

US 13, I keep telling him he'd be even taller if they hadn't turned up so much for feet.