If your company is using the Microsoft Online Services Directory Synchronization tool and you’ve a very large Active Directory directory service or very large distribution groups, directory synchronization may fail with the “Maximum Request Length Exceeded” error message in the Event Viewer. The number of objects in a batch can be set by a registry key. To edit this registry key: 1. On the directory synchronization computer, run RegEdit.exe and navigate to HKEY LOCAL MACHINE\Software\Microsoft\MSOLCoexistence. 2. Add a new DWORD value named “ExportBatchSize.” 3. Set the value to less than 20 and test synchronization again. 4. Continue reducing the value until synchronization succeeds.

More info: Windows registry information for advanced users