type "print" and press return to see the size of the partition, then you need to resize it (from the above link):
To resize the partition, use the resize command followed by the minor number for the partition, the starting place in megabytes, and the end place in megabytes.
resize 1 1024 2048
This example is increasing the size, which is the opposite of what you want to do. You want to shrink the size. The print will show you the ending location, you need to use that for the second argument.