I tried various actions, like Control-click, but there is either no menu or the menu does not provide a command to change the format or size of the cells. I looked at the protocol for the CrossTabBox object. The only messages that might help are << Set Height and << Set Width, but these messages had no effect.
Maybe someone else knows of a solution. If there isn't one, then I suggest that you enter a request in the JMP Wish List. (Be sure to search first because someone else might have already entered such a request and then you can vote for it.)