See the following Wiki on how to extend VHD`s with Hyper-V (or VMM).
You have to complete the required operations within the guest OS afterwards.
http://social.technet.microsoft.com/wiki/contents/articles/266.how-to-increase-the-vhd-size-in-hyper-v-r2.aspx
Kristian (Virtualization and some coffee: http://kristiannese.blogspot.com )