How to Calculate size of AD (Active Directory Databse)

Before you can install Active Directory, you will also need to know the potential size of the Active Directory database. Active Directory space requirements per object are much smaller than you might think. The approximate sizes of objects and attributes in Active Directory are as follows:
• Security principal (User, Group, Computer) = 3,600 bytes
• Organizational unit (OU) = 1,100 bytes
• Security certificates mapped to a user = 1,500 bytes
• Object attributes = ^100 bytes
• Access Control Entry (ACE) = 70 bytes per ACE

FSMO Role

There are five FSMO rore that work with on windows server 2000.2003, 2008, among them two roles work as forest wise and three domain wise

Forest wide FSMO Role

Schema Master: The Domain controller that hold SM Role responsible for changes in schema properties and Attributes.
Domain Naming Master(DNM): The DC that hold DNM responsible for add and remove domain controller in the forest.

Domain Wide FSMO Role:

PDC Emulator:

Relative ID(RID) Master

Infrastructure: