A. If you're planning out a new AD implementation and need to estimate the size of the AD database for the purposes of sizing (particularly the amount of RAM required for domain controllers), Microsoft has the Active Directory Sizer utility. The utility lets you input information about the environment and gives an estimation of the AD database size, in addition to the number of domain controllers and global catalogs required.

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The utility is wizard-driven and asks numerous questions about the environment. It can also estimate number of logons expected and other factors. An example of the utility's output is shown here.

The tool can be downloaded from Microsoft's site.

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