By default, an email message you could send or receive in Office 365 is 25 MB. However, you can increase it up to 150MB. Here is how.
- With Office 365 admin center open, go to Exchange admin center.
- Click on recipients.
- Highlight the display name which you want to increase the email message size. Then click on 3 dots… Select Set default message size restrictions.

4. Enter the size of sent/received message no more than 153600 KB.

5. Click OK to save the settings.