"No checks of $100 million or more accepted "

I guess it's a printing and legibility thing: How small the type/script or how big the piece of paper in order to print how legible a check with a line saying
"One hundred twenty three million, four hundred fifty six thousand, seven hundred eighty nine dollars and 00/100 cents"

I suppose you could deduce for us what the implicit printing standard might be. I'd be curious!