...or do you mean the size of the partition on the hard drive on which Windows XP is installed?
If you are using the Boot Camp partition as for booting to Windows, the only way you can change the amount of memory is by installing (or removing some of) more RAM physically on that Mac.
If you have 4 GB physical RAM and Boot Camp shows 3 GB, bc will still show 3 GB.
If you are using the Boot Camp with Fusion or Parallels, in that case change the RAM amount used from the VM's preferences in that application.