Volume Expansion

How large a volume can be expanded in Windows depends on the cluster size. The cluster size is set when in initial volume is set and can not be changed. Here are a couple of articles that discuss this.

Maximum NTFS Volume Expansion

How can I view the allocation unit size of a NTFS partition in Vista?

You have to set the new of megabytes for the expansion to one megabyte (1 MB) less that the theoretical maximum number.