How to set message size limit for one particular user from Exchange admin center?

Step 1: Sign into Office admin portal via https://portal.office.com/

Step 2: Click on Admin and navigate to Exchange admin center.

Step 3: In Exchange admin center, navigate to Recipients > Mailboxes.

Step 4: Select an user from the list of users.

Step 5: Click on Message size restrictions button on the top.

Step 6: Set the maximum sent and received messages size limit and click Save.