PS: it can't hurt to make a virtual HD of 30GB or larger even....
I would simply create a new VDI hard drive.
The VM's hard disk will only use what's required so will "expand" as more stuff is added, so as
rubes advised, a 30GB drive would be more than adequate as your install will only take a percentage of that (and PCLinuxOS won't see the VM/VDI as a 30GB file but only the percentage used)