Windows 10 – Windows Live Mail – How to reverse sudden reduction in type size when drafting new messages

Windows 10 is working well for me - and so is Windows Live Mail, except that every so often, for no apparent reason, although I am still apparently writing in Times New Roman 12pt font as intended, the type size of my draft email suddenly shrinks and is barely legible. Perhaps I have accidentally hit a key? If so, how do I reverse it? When I review the sent messages, and received messages, they look normal. The problem is only with new messages as I write them.