Shutdown the Virtual Machine.
In the VirtualBox Manager, select the machine, click Settings -> Storage.
Select Controller SATA and click on the Add Hard Disk button:
Leave VDI (VirtualBox Disk Image) selected and click Next.
Leave Dynamically allocated selected and click Next.
Type in the full path or choose through the browse icon the Location for the disk file to be created.
Type in 50.00 GB in the Size field and click Create.