- go to TB menu > Settings > General > Language & Fonts > Fonts > click 'Advanced' button

- at the top, set 'Fonts for:' to Latin if it isn't already

- change Proportional and Monospace fonts/sizes to your liking

- set Minimum font size to 14

- at the top, change set 'Fonts for:' to Other Writing Systems

- repeat the process of changing fonts/sizes

- at the bottom, make sure 'Allow messages to use other fonts' is disabled if desired > OK > close the popup

- set TB menu > Settings > Composition > HTML Style to Variable Width and Medium font

Thanks very much for that Ed.

It worked very well on simple text emails, but ones with HTML content still have tiny tiny fonts. Can I do anything to fix that please?

Thanks very much for that Ed. It worked very well on simple text emails, but ones with HTML content still have tiny tiny fonts. Can I do anything to fix that please?

'Allow messages to use other fonts' is disabled?

Yes, it is disabled. :-)

putting Message body in "simple HTML" makes it large, but all formatting then gets lost.

Try increasing the Minimum font size for both options. Set it to the same as you set the other sizes.