Changing sprocket sizes: will front derailleur still work?
According to the Shimano Tech Doc it looks like 48 teeth is the maximum recommended size and 48/38/28 is the recommended chain ring combination.

The spec sheet also indicates that the maximum jump between rings is 10 teeth, so even if you could fit a bigger ring, you would have to replace the other two as well.

I believe the Kaitai normally comes with a standard 11-32 rear cassette.  Normally 11 is the smallest number of teeth that you will find, but Shimano makes a cassette model called Capreo that you may be able to find and use that goes as small as 9 teeth if you are trying to get a "bigger" gear.  The nine speed cassette ranges from 9-26T, so you trade the bigger gear on the low end for a gear that will be much harder to pedal up hills.