Hi Jason,
We don’t need to manually split the contents in the public folders stored in Exchange Online, since Exchange Online will detect the quota usage of the public folder mailbox and automatically split the public folders when the public folder mailbox reaches the issue warning quota value (24.5GB).
When a public folder mailbox reaches the Issue Warning Quota value of 24.5 GB, this process is automatically triggered to redistribute where your public folders currently reside. This may result in Exchange Online simply moving some public folders from the nearly-filled public folder mailbox to another pre-existing public folder mailbox holding less content. However, if there are no public folder mailboxes with enough free space to move public folders into, Exchange Online will automatically create a new public folder mailbox and move some of your public folders into the newly created public folder mailbox. The end result will be all public folder mailboxes being below the Issue Warning Quota.
Public folder moves from one public folder mailbox to another are an online move process similar to normal mailbox moves. Due to the move process being an online experience your users may experience a slight disruption in accessing one or more public folders during the completion phase of the online move process. Any mail destined for mail enabled public folders being moved would be temporarily queued and then delivered once the move request completes.
For more details, please refer to the “How is public folder management in Exchange Online performed” part in Public Folders and Exchange Online.