how do I change the partition size

You can use Gparted to resize partitions, but you can't resize a mounted file system. You'll need to boot off the LiveCD or LIveUSB and run Gparted from there (already installed on the LiveCD/USB).

Note that if one of the partitions is NTFS then you should run chkdsk and defrag from Windows on the NTFS partition first to make sure that there are no problems with it, and that the move operation has the least amount of work to do (can be nasty on a badly fragmented partition).

It should go without saying that you should also make sure that all of your important data is backed up before proceeding with _any_ of these operations. They can all result in major data loss in the case of a power failure, hang, or other error.

Chris.