I'm running a Windows 7 Ulitmate 64 bit vm on my 15" MacBook Pro (Retina Display). I've assigned the vm 1 processor core and 2 GB RAM. Its virtual HD is 64.2 GB (in Fusion "Preference>General", but in Fusion "Preferences>Hard Disk" the slider is set to 40 GB. In "Preferences>General" there's no reclaimable space. Looking at the Windows 7 control panel, I see that there's barely 1 GB free of my 40 GB drive.
The SSD drive on the Mac is 250 GB and has 67 GB free, so I assume I could up the vm's hard drive space to 60 or 70 GB and not cripple the Mac OS, and that I can do that by shutting down the VM, deleting snapshots, and then upping the virtual HD size with the slider before rebooting.
By the way, here's the message I'm getting from Windows at boot. Curiously, it comes only after I've logged in to my account, not as part of the boot process, and it reappears if I log back in. I don't have another user account on the machine, but I'm assuming the problem stems from the virtual HD being almost full: