The hardware requirements can are essentially the same as CRM 2015 -
see here.
If you're installing AD and SQL server on the same VM, you should add the appropriate resources needed for those.
I made a
video some time ago on installing CRM 2013 into a VM which may be useful. I think I assigned 4GB memory, 2 processor cores and around 100GB disk space to the VM.
Neil - My CRM Blog