Wil van Antwerpen wrote: Then go into Fusion menu "Virtual Machine", "Settings", "Hard Disk" and increase the size.
On starting your guest OS, the virtual disk will be bigger.
While that is true nonetheless that is only part of it and the additional space the virtual disk now has needs to be partitioned/formatted or the existing volume expanded/extended in order to use the additional space.
@maxschle Have a look at, Resizing a virtual disk in VMware Fusion (1020778) or an older document but good as a reference, Resizing Virtual Disks With Step by Step Instructions.
Note that the above links are for normal file based Virtual Machines not the Boot Camp partition run as a Virtual Machine. If you need to increase the size of the Boot Camp partition you'll need to do that with Boot Camp Assistant (which has nothing to do with VMware Fusion) and you should backup the Boot Camp partition with Winclone before resizing the Boot Camp partition. You'll then need to delete the Boot Camp icon in the Virtual Machine Library and add it back. Note that in this case the Boot Camp icon in the Virtual Machine Library is for the meta-data Virtual Machine used to run the Boot Camp partition as a Virtual Machine not an imported Boot Camp partition that is now a normal file based Virtual Machine.
Also in the OP you said "I plan on only using Windows so I would like to have as much HD free as possible." and if that's the case when why even purschase a Mac!? 