Which font settings did you change?

Your plain text messages may be displayed using a specific (usually monospaced) font and maybe you haven't set this yet. It hides behind an "Advanced" button. And there is a useful "minimum size" setting which should meet your needs.

http://xenos-email-notes.simplesite.com/417754775

In Display I changed the default font from Ariel 18 point to Ariel 30 point without effect. Since reading your reply I altered the minimum size setting to 12 point also without effect.

In Composition the only available option is HTML (no plain text option available,) which is set to Variable Width, Large.

Unless there are other settings you know of I shall work through the links you kindly gave to seek a solution.

Those settings are in pixels, not points, and if you're using a modern high resolution display the numbers would need to be larger. Try something bigger, say 18, for the minimum.

No, my screen is small and old, nevertheless changing the minimum size to 20 solved the problem of legibility for me. Many thanks.

There is a near-identical set of settings in Firefox too.

As an aside, I would advise you to leave composition size settings on "medium". Some users increase the composition font size to help with their own reading when writing an email message, not appreciating that doing so will make their text enormous on the recipient's computer. "Medium" means that it will display on the correspondent's computer at their default and accustomed font size.

Thanks for the tip. I have changed composition size to medium as advised.