Create specific sized partition

Hi All,

We are trying to create a specific provisioning template:
Due to our software requirements we need a D: drive with 20 gigs, and the OS drive to be left with all the leftover space.
The thing is our machines in the environment have a HDD’s with a different size, that’s why we can’t create a template with fixed partition size.
Does anybody know how this can be achieved, e.g.  leave 20 Gb of space unused when creating the C: drive?

Thanks