Using New Outlook on Windows for professional communication and productivity
okay I think I figured it out - at least on office 365 using a Macbook Air. I opened an email. Across the top is a bar with various commands. One of them is zoom. When I opened it, it was set for 67%. I reset it to 100% - problem solved. I think clicked the reset button and it also stayed at 100%. When I checked on other email they were also all at the normal size.
Hope that helps.