First try the drive manufacterer's utilities.
If they don't work, the "ultimate boot cd" has several disk utilities on it that can do it. Many of the programs are brand specific, but work if one of thier drives are present. I have used it for exactly this purpose (return a 60g to 60g from 30g). I think ranish was the program I used.
Here
http://www.storagereview.com/guide/guide_index.html is a place to read up on how the disk capacities can be manipulated.
These are low level changes to the disk and require a floppy or cd boot disk .
After that is changed you will probably still have to fdisk and format for use.
Jim