That sounds like the size of Virtual Machine not the VMware Fusion install. Your virtual machines can reside on any drive, in fact I prefer external drives for mine.
If you start Fusion and bring up the Virtual Machine Library Window you can find where your Virtual Machine is stored. Just right click on a machine and select the show in finder command.
Once you find the VM_name.vmwarevm bundle copy the whole thing to your external drive. Once you have started it on the external drive and confirmed everything is working fine it is OK to delete it from the internal drive.