Please follow the below guide if you think you need (or have been told to) clear down the size of your mailbox.
Option 1: Compact the OST file to reduce the size.
Outlook has the option to compact the OST and PST files to reduce their size. The steps are as follows:
Delete any items that you do not want to keep and empty the Deleted Items folder.
Click File > Account Settings > Account Settings.
Under the Data Files tab, select the OST data file that you want to compact
Click Settings> Advanced > Outlook Data File Settings…
Click Compact Now and then click OK.
This will compress the OST file to an extent and may resolve the issue temporarily.
Option 2: Clean up the mailbox
To fix the issue for a long time, To reduce the OST file size, you can choose to clean up your mailbox. However, this method requires you to delete older emails. The steps are as follows:
Click the File> Info and click on Cleanup Tools beside Mailbox Cleanup
Click on Mailbox Cleanup… option.
You can view your current mailbox size stored in the local OST file and on the server by clicking on the ‘View Mailbox Size…’ button.
Enter the number of days to find the older emails and delete them to reduce the size of the OST file and mailbox on the server.
You can also Find items larger than, the Empty Deleted Items folder, View Deleted Items Size, View Conflicts Size, or Empty Conflicts to reduce the OST file size.
Further, you can look for large emails, download all the attachments, and then remove the attachments from emails to reduce the OST file size.
PLEASE NOTE, IF THESE 2 METHODS ARE NOT SUFFICIENT, FURTHER CAN BE ADDED IF REQUIRED.