How do I find the size of Source Public Folders?
The simplest way to determine the size of your Source Public Folders is to run a PowerShell query to obtain the size-related statistics from your Public Folders.
Here is an example script that will pipe the results into a file named PFStatistics.csv. You can then use this information to break up your Public Folders into smaller-sized Public Folders.
Get-PublicFolderStatistics -ResultSize Unlimited | Select-Object Name,FolderPath, AssociatedItemCount, DeletedItemCount, ItemCount, TotalAssociatedItemSize, TotalDeletedItemSize, TotalItemSize | Export-csv .\PFStatistics.csv -NoTypeInformation