You are currently using 257 MB (11%) of your 2428 MB.Once I got to 11% of my 2GB allotment, Gmail started increasing my maibox size to keep my allotment there (why 11%? prime number? symmetry?).
So it's now 2.5GB and still growing.
Update 10/5/09: It's 7GB now, but I use it faster than it grows.