Many of the hybrid customers wants to streamline there Message Size limits to match their Global settings on Exchange On-Prem environment.
Microsoft is constantly making changes to these settings & keeps on increasing it, earlier it was 35 Mb & now its being changed to 150 MB.
Current Exchange Online limits can be referred from below official link.
This limit is huge expand from existing limits & is not good for many organizations as sending/receiving huge size emails causes more issues than it resolves.
Especially for Hybrid environments keeping the same message size across whole Messaging environment is many a times required by corporate policies.
In on-premise world you can just set the Global transport configuration settings that can control the Message size limits but with online world you have to do below.
If you are using GUI than just select the mailboxes -->Set default message size restrictions.(25 MB example)
For setting this thru exchange Online Shell use below commands.
Get-mailbox -resultsize unlimited | Set-Mailbox -MaxSendSize 25MB -MaxReceiveSize 25MB
This will set each mailbox in the online environment with message size limits of 25 MB.
For new mailboxes created in the environment either you can make a script that does this on regular basis for new users or just update the mailbox dial plan.
Get-MailboxPlan | Set-MailboxPlan -MaxSendSize 25MB -MaxReceiveSize 25MB
Majority of our customers are using this approach so that when asked by users about message size limits a consistent answer can be provided rather than stating different answer for online user vs on-premise user.
Thanks for reading