This was a question from large customer in the southwest who has over 250,000 users to sync to Office 365. AAD Sync requires a Full SQL Server to handle anything over around 100,000 user objects.
Here are the steps to install AAD Sync with a Full SQL Server:
0) Install a SQL Server 2008 R2, 2012, 2014 on another server
1) Grab AAD Sync tool here.
2) Run MicrosoftAzureADConnectiontool.exe on the AAD Sync Server (VM or Physical)
3) Close AAD Sync wizard once it opens to avoid installing to a SQL Express DB
4) Open a command prompt with Administrator rights
5) Change to the c:\program files\Microsoft azure ad connection tool\ directory
6) Run the following from that directory:
DirectorySyncTool.exe /sqlserver <SERVER NAME> /serviceaccountdomain <NETBIOS NAME> /serviceaccountname <AD service account name> /serviceaccountpassword <password>
directorysynctool.exe /sqlserver sql.contososchool.edu /serviceaccountdomain CONTOSOSCHOOL /serviceaccountname AAD-SyncSVC /serviceaccountpassword P@ssw0rd1
7) AAD Sync Wizard will automatically open and allow you to follow the wizard to complete the configuration. See more on wizard here.