1. Add a new virtual hard disk from within the Boot Camp VM settings (it should be Add Device… —> New Hard Disk) of IDE type (the size doesn’t matter, and probably it shouldn’t be of the split type), and save it – with a NameOfVMDK.vmdk name of your choice – inside your Boot Camp VM bundle (it should be the default position in the save dialog); close the VM settings and quit Fusion.
2. In the Terminal, type this command:
$ /Applications/VMware\ Fusion.app/Contents/Library/vmware-rawdiskCreator create /dev/diskX fullDevice ./Desktop/NameOfVMDK ide
… which will create the new .vmdk file on your Desktop.
3. Copy the newly created .vmdk into your Boot Camp VM bundle, thus overwriting the existing one (which was created from within the VM settings).
4. Open Fusion and the VM settings again and verify that the new IDE virtual disk now has the size of your real added hard disk.