Resizing a Windows Server guest

I need to implement a Windiws Server 2008 R2 guest VM acting as a Remote Desktop Server.

At the moment I can't predict how much the VM will be used, including the number of users and the way they will use the VM.

I am considering creating a Windows VM with an arbitrary number of vCPU (let's say 2) and an arbitrary amount of vRAM (let's say, 8 GB).

I know that if I see that the VM gets overloaded (using Microsoft Performance Monitor) I can add more vCPUs and increase vRAM size.

Is there any guideline to change the resources assigned to a VM?

I mean, given that Windows allows adding CPUs and RAM, is it advisable to start with a minimal configuration and to increase it or to start with a larger configuration and recduce it?

Regards

marius