To increase size of virtual disk, follow Resizing a Virtual Disk for Windows Guests with Step by Step Instructions.
Other options include creating a second disk and storing your user data there, if they are the reason for the big size (the original disk would be just for Windows files and programs), or storing the data on shared folder (again if user data is what is the big thing).
To create a second bigger disk and move everything to it, you would have to use some specialized tool just like on a normal Windows PC with 2 drives - I think Norton Ghost could do that (sure others could do it as well, just this one came to my mind first).