In this age of virtualization I typically try to size a Windows system volume to a realistic size. That is, a size that will provide a good cushion, but not over allocating space. Occassionally, I do screw up and I need extend the volume. Now, one cannot resize a live system volume with diskpart, even if you have sufficient space on that disk. With additional data type volumes, one can add the space necessary to the LUN, run diskpart and extend the volume.

To do this with a system volume, we would utilize a PE (preinstallation environment) disk. I use Bart PE for this. BartPE doesn't start disk management services automatically. Here's how you would extend a Windows system volume, provided you have space on the disk and provided you want to fill that extra space.

1. Boot the machine with a PE (Win or Bart)
2. Get to a DOS prompt
3. Run net start dmserver
4. Run net start dmadmin
5. Run diskpart
6. Run list volume
7. Run select volume *insert your volume number from list above*
8. Run extend
9. Your volume should be extended successfully

I hope this helps you out!