I recall no font requirements in the original instructions and the OMB form requires customization to insert the bank info. That said, I would follow the "clear and conspicuous" disclosure requirements we are used to. So avoid a 6 point font but increasing the size or decreasing it slightly either way shouldn't be an issue, IMHO.
My opinions are not necessarily my employers.
Rules and Regs minus Relationships equals Resentment and Rebellion. John Maxwell