How much RAM should I give my domain controller ?

I recommend installing physical and virtual DC’s. The virtual DC’s should have the same resources as the physical DC’s. This is in case your physical DC’s fail and your virtual DC’s have to take over. Remember if you use virtual DC's time sync is really important so make sure you have NTP setup.

This documentation from Microsoft states that calculating the RAM requirements for a domain controller is complex because it will automatically adjust if it doesn’t have the required resources. However they recommend a basic plan to calculate the required RAM for domain controllers based on the following variables:

Base OS RAM requirement + NTDS.dit size + SYSVOL size + Windows applications + 3^rd Party applications

NTDS.dit file is in C:\windows\system32
SYSVOL can be found by going to start -> run \\domain.local\SYSVOL

Have a look at the appwiz.cpl (installed applications), services.msc (that are started), the scheduled tasks and the process currently running on the system. Also look at users logged in by RDP do they leave MMC's open often ? You can visit 3rd party vendor sites or contact the support for the RAM requirements.

Once you have done your calculation round up to allow for some wiggle room. If your calculation comes out as 3.5 GB, round that up to 4 or 6 GB.

Sample calculation:
Base OS requirment - Windows Sever 2008 R2: 2 GB
NTDS.dit: 1 GB
SYSVOL: 1GB
Windows apps: 1 GB
3rd party aps: 1 GB

Total: 6GB