Changing medium to large is an HTML process. You might first consider why you want recipients to receive your emails in a large font. Some may find it offensive, as they may prefer to set their own font size. Your replies are also in large font when replying in HTML. You may need to change settings to always send HTML, regardless of style changes. When you receive messages, are they of acceptable size? That is, my real question is whether you want recipients to see messages in a large font, or is it just for yourself to see messages in large font? This may seem picky, but the solutions are different for each. Thank you.

Thanks, David. There are 6 sizes to choose from when I look at the HTML settings - Tiny, Small, Medium, Large, Extra Large, and Huge . I only want to go from Medium, which I believe is default, up one to large. This makes it easier to see what I am typing. When I have sent a new email with the slightly larger font, replies from the recipient maintain that slightly larger font in my portion of the thread, which is helpful as well, but usually not in the reply. I assume that the recipient also sees the slightly larger font size. All this seems acceptable to me and I assume to the recipient. You asked if incoming messages are too small. They usually come in at the slightly smaller font size (Medium) that I started with. It would also be nice to view these incoming messages in the slightly larger font size. Given the size that my (older) eyes need, I would like incoming messages to also appear slightly larger (Large). I hope this all makes sense! Thanks again. Look forward to your response! Rick

Ok, check this earlier response I gave:

  https://support.mozilla.org/en-US/questions/1437527  

in summary, I encourage medium font as that allows recipient to pick their own. The other settings ensure that you see your desired font whether sending or receiving, whether HTML messages or plain text. Follow the steps in detail about the Latin and Other writing systems and this has always been a 100% success. If you have difficulties. please let me know. As I am also not as young as I'd like to be, I respect your issue. :)