Ways to manage your mailbox size (Windows)

1. In Outlook, choose File> Tools > Mailbox Cleanup.

2. Do any of the following:
   • View the total size of your mailbox and of individual folders within it.
   • Find items older than a certain date or larger than a certain size.
   • View the size of your Deleted Items folder and empty your Deleted Items folder.
   • View the size of your Conflicts folder and delete this folder.

Here are some ways to keep your mailbox size under control:

• Empty the Deleted Items folder – Empty the Deleted Items folder frequently to make sure you aren’t keeping messages you don’t need.
• Store attachments outside your mailbox – As attachments accumulate over time, they can take up a lot of space. For attachments you want to keep, consider saving them to your OneDrive, to folders on your computer, or to a SharePoint site.
• Empty the Junk Email folder – Periodically empty the Junk Email folder to save your inbox space for messages you want.

Over time, the Junk Email folder grows larger as new messages are identified as junk or spam and moved to the folder. Periodically, you should empty the folder so the messages aren’t increasing your data file size or using your mail server storage quota.

Tip:  Review messages to ensure that messages that you want to keep weren’t mistakenly moved to the Junk Email folder.

1. In Mail, in the folder pane, click the Junk Email folder.
2. Click Folder > Empty Folder.

To permanently delete messages from the Junk Email folder without emptying it, select one or more messages, and then press Shift+Delete.

Reference: Manage my mailbox size - Microsoft Support