Here’s how to change the Office 365 Message Size Limit for Messages, Tasks etc.. for Individual Accounts and as the default for new Office 365 Accounts.

Change Office 365 Message Limit for an Individual Account Only

Step 1. Open Exchange Admin

Once logged in to your Office 365 portal click on Admin on then left menu bar and then Exchange to open the Exchange Admin

Step 2. Open the Recipients Mailbox Properties

Click on Recipients in the left side bar, click on the Individual Mailbox and then click on the edit icon.

Step 3. Change the Message Size Limit

Click on Mailbox Features and then scroll down to Message Size Restrictions.  Click on View details

Step 4. Change the Maximum Message Size

Change the Maximum Message size up to a maximum of 150000KB for Sent and Received message

Change Office 365 Message Limit for New Accounts

Step 1. Open Exchange Admin

Once logged in to your Office 365 portal click on Admin on then left menu bar and then Exchange to open the Exchange Admin

Step 2. Change the Default Message Size Limit

In the mailbox view click on the … and then click on Set Default Message Size Restrictions.