Is it original wiring? (K&T?)
Only reason I ask is if it is K&T, I'm wondering how well a GFI would
deal with the neutral, seing as how one neutral typically served many
loads. Maybe wouldn't make a difference. Tom Horne, where are you?
Your options are as Travis listed (unless again, it's original wiring,
and you have a fusebox and therefore no breaker to replace); or to cut
out some plaster and install a larger box.
I think we need to know more about what state your wiring is in before
any other options can be offered.