Apple mail app, how do I Increase (zoom in) displayed text size (not font).

My wife (a Mac book air user) has been struggling with the mail compose window. I have tried to find a way of increasing the size of the displayed text without increasing the font size, which changes the size of the text for the recipient. I've searched the forums with no luck, finally resorting to calling Apple support again today but there doesn't seem to be a solution. Has anyone resolved this issue? Why don't Apple address the issue?

Just to make sure I wasn't going mad, I checked in Thunderbird on my Ubuntu Linux machine and on my work PC running Windows 10 and using Outlook, no problem. I can zoom in and out with Command opt + and - (ctrl+alt on Linux/Windows of course). Is it time to give up on Apple mail software and install Thunderbird, it seems far better.

Any help would be greatly appreciated.