Hi Robin,
The maximum message send size for Office 365 is 35 MB. Based on my experience, it's same as OWA and the size limitation can't be extended.
Recently, Microsoft has received reports of an issue in which some customers using Mac Outlook or Entourage may be unable to send messages exceeding a size of 6 megabytes. Microsoft is working to resolve the issue. If you have this issue, please don't hesitate to let me know.
@AppRiver, thanks for your information.
Thanks,
Aaron Yuan