Hello Rin,
Sorry for the delay in response, the text size of the email messages depends on the font settings and the person who sends the emails. currently we have an issue with font settings on MSN client please regret the inconvenience caused. Our support engineers are working on it to resolve the issue.
If you still have issue with font settings please wait for an update from MSN and that should resolve the problem automatically.
For more information you can go to below link and contact MSN technical support team.
https://support.microsoft.com/en-us/contactus/msn/explorer-tech-services
Thanks and regards,
MSN Support Team.