I'm sure the highest number answer is 9^9 as stated before or possibly 9E9. But as DKWicked also noted, you can use 99!. Following that pattern you can also use ((99!)!)! etc. So, using two numbers and any combination of other symbols, there is no largest number.

Unless you're talking about physically largest and not numberically largest, then it would be 88.

::edit::

OR!!!!

FF, if you're using Hexidecimal.

Or or ZZ if you're usin base26.

Or.. ZZZZ (base 50somethin?)

Anyhow, as you can see, there's not really any right answer to that question.

What is it they say about silence being golden?