You could probably get away with a 48 tooth with a Front Derailleur that was shifting a 42 but it is touch and go much bigger as tjspeil pointed out. It may work but you will be havign a lot of issues with the chain dragging the FD with certain gear combination's.

I found that a 26/36/48 is a pretty usable crank set up on my bikes but I am running 700 wheels.

Honestly the best thing I ever did for my commute was quit trying to make a road bike out of my Mountain bike and got a touring bike.