Hi,@Roger Roger
Thanks for posting your question in the Microsoft Q&A forum.
Based on your description, I will answer your questions in turn.
1. Is it possible to extend the mailbox size beyond 100GB?
The size of a shared mailbox is usually limited, and the standard mailbox size limit for an E5 license is 100GB. Currently, it is not supported to expand the standard mailbox size beyond 100GB.
2. How much archive mailbox size is supported with an E5 license?
For E5 licenses, you can enable unlimited archive mailboxes, which means that the archive mailbox can automatically grow as needed without a specific size limit.
3. If I need to enable archiving, do I need to create any policy?
To enable an archive mailbox, you need to create and apply an archive policy in the Exchange admin center.
4. If I enable archiving, will it degrade Outlook performance?
Enabling archiving does not significantly affect Outlook performance because messages in archive mailboxes are stored in the cloud rather than on the local client.
If my answer is helpful to you, please mark it as the answer so that other users can refer to it. Thank you for your support and understanding.