1/grahams number would be vastly smaller than a Planck length, which is supposedly the shortest/smallest distance that has any meaning (about 10^-20 the size of a proton), so I guess font size couldn't be less than 1 Planck length. And there aren't anywhere near enough Planck units in the entire universe (around 10^186) to make a scratch on grahams number.
But if grahams number is that big, imagine what grahams number raised to the power of grahams number might be?? Then imagine this - there are still an INFINITY of numbers even larger than that!