Hi Peter,
Welcome to our forum community. Thank you for posting here. Normally this issue is fixed by the steps specified in the article attached below depending on your outlook version: Change font sizes in Outlook for Mac - Microsoft Support.
Additionally, you contact Outlook for Mac support. Please refer to this article on how to contact support: Contact support within Outlook for Mac - Microsoft Support which the support team designated to deal with issues of Outlook for Mac as our main focus in this community is on Microsoft 365 Business Exchange Online and Mail flow issues. I believe you will get more accurate assistance regarding this issue .
Thank you for your cooperation and understanding. Please do not hesitate to post your queries in the Microsoft Community, and we will always do our best to assist you.
Sincerely,
Albert | Microsoft Community Moderator