Scam_Stalker A virtual machine is not a copy of your computer; rather it is a "virtual" computer. A program (VirtualBox, VMWare, etc.) emulates a computer. From within the virtual machine, it behaves like a new, separate computer. The scammers won't be able to find banking details.
How much space does a VM take? It depends on what you install. A Windows installation takes about 20-30 gigabytes. The "hard disk capacity" setting when creating a virtual machine is only a maximum limit unless set to allocate all disk space immediately. The virtual machine file grows as more space is used up by the virtual machine.
Hopefully this clears up your questions.