Hyper-V adds the ability to increase the maximum size of a dynamically expanding or fixed-size virtual hard disk.  To do this you need to open the Edit Virtual Hard Disk Wizard (by selecting Edit Disk... from the Hyper-V manager), select the virtual hard disk you want to expand, select Expand on the Choose Action page and then enter the new size of the virtual hard disk that you want.

Three things to note:

• You can't do this to a virtual hard disk that is associated with a running or saved stated virtual machine.
• You shouldn't do this to a virtual hard disk that is associated with a virtual machine that has snapshots (as you will invalidate the snapshots).
• After expanding the virtual hard disk there will be an empty space at the end of the virtual hard disk.  You will either need to create a new partition to use the new space, or expand an existing partition into the new space.

Source:→ MSDN

Virtualization, Hyper-V, Windows Server 2008, VHD, Virtual Hard Disks