The nice thing about HyperV is that if you screw up and pick a lousy size for a C drive, you can easily make it bigger
Expanding Virtual Hard Disks with Hyper-V:
When figuring out the size of a parent there’s the C drive of the parent to worry about on the physical disk. It should be big, but not too big. After all it’s just going to be Windows OS as a HyperV role. Nothing else will be installed. So it’s base OS + patch bloat.
I have a three year old Win2k8 r2 with an 88 gig c drive with 37 gigs free as a guideline of how much you kinda want to have. 50 gigs bloat growth over a three year period. These days it’s not growing that much, but still it’s kinda worrying that 50 gigs it merely for a parent for the important servers.
If you lay down an SBS 2011 remember you can move SharePoint/Exchange over to another drive. I think 80-120 gigs comfy on SBS 2011 standard as the C drive with as much moved over to other drives.
For the exact how to expand drives in HyperV follow:
to give yourself more breathing room.