Exchange 2013 - New mailbox database sizing requirements

We want to migrate our Exchange servers to a new environment and get off some storage that's about to have its support expire, we are currently in a hybrid (On-Prem - O365) environment, so we have moved 95% of our mailboxes to the cloud already.

Regarding freeing up whitespace, I think the easiest thing to do is create a new mailbox database and move the remaining mailboxes to it, instead of taking it offline and defragging, correct?

How can I find out how large the new mailbox database 'should' be? Is it just a matter of exporting a list of all on-prem mailboxes and their sizes and base the new database sizing on that total?

Thanks in advance.
Christian HansUndecided... Asked:
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Todd NelsonSystems EngineerCommented:
Regarding freeing up whitespace, I think the easiest thing to do is create a new mailbox database and move the remaining mailboxes to it, instead of taking it offline and defragging, correct?

That's correct.  However, how you deal with it can also depend on how much data has been left on premises.  If the data size is negligible, then an offline defrag won't be an issue.

Check your current DB size using this command...

Get-MailboxDatabase -Status | ft -auto Name,DatabaseSize,AvailableNewMailboxSpace

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How can I find out how large the new mailbox database 'should' be? Is it just a matter of exporting a list of all on-prem mailboxes and their sizes and base the new database sizing on that total?

Use the command I provided above.  You cannot set the DB size but you can set how much storage is available to the server that will host the DB.  Use the following reference for DB size best practices based on stand-alone or HA.

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