Not sure what you mean by the 'box survey contents'. Do you want to increase the font size for message bodies, the message list, the menus, or everything?

https://support.mozilla.org/en-US/questions/1269361

Thanks for a quick response. I meant everything on the Thunderbird start page incl. the content of the selected message box (= body?). I use Firefox and Windows 10. Your answer helped me reanalyzing the problem. My solution was increasing the Example text in Windows 10 Settings.

Kind regards