When reading or composing a message in Outlook for Mac, you can use the pinch or spread motion with your fingers on a trackpad to reduce or enlarge text in a reading or composing window. More info on trackpad gestures.
When composing a message in Outlook for Mac, press Control+Command+Z to open the Zoom pane o adjust the percentage up or down.
For more information on options, see Use a screen reader to zoom in or out in Outlook