After useing system restore my System drive has doubled in size.
How can I delete the system restore data to save space?

if your restore works fine then right click "my computer">properties>system restore>here turn off system restore (this deletes all points) then turn it on again and you should be fine

It is best to reboot in between turning it off and on. Then when turned back on create a restore point.  
 

yes it is best to reboot in between

Use the Disk cleanup tool to delete all but the most recent restore point:
 
To delete all restore points except the latest one, use the Disk Cleanup utility. Click Start, All Programs, Accessories, System Tools, and then Disk Cleanup. Click on the more options tab and then select Clean up in the System Restore dialog box.
 
.. or set the amount of space available for restore points to a smaller value:
 
Select Start, then Control Panel and double-click the System icon. Then click on the System Restore tab on the dialog box. Depending on your disk setup, do the following:
 
• Single partition: Adjust the space system restore uses on the disk by moving the slider left to decrease space usage, or right to increase space usage. The default maximum space usage is 12%.
 
• Multiple partitions or multiple disks: Click on the drive you want to adjust in the available drives section on the System Restore page and then click the settings option. You can then adjust the space system restore uses on that drive by moving the slider to the left to decrease space usage, or right to increase space usage. The default maximum space usage is 12%. Repeat for each drive as necessary.