While Mozilla Firefox at last has gained a preference which allows to set Default Zoom level, Thunderbird still haven't got it. This is significant accessibility issue.
There exist a hack which allows to set the desired zoom level using Browser Console in Firefox, it is described here. I supposed that similar approach would work in Thunderbird and found the working command:

ZoomManager.setZoomForBrowser(getBrowser(), 1.2);

If this command is run in Error Console (analog of Browser Console in Firefox and it can be launched by the same shortcut Ctrl+Shift+J), then 120% zoom will be applied to the letter preview pane (at least).
Now the problem is to run this command at Thunderbird start. I installed userChromeJS extension which is developed to support Thunderbird version 91+ and which will "load the contents of chrome/userChrome.js from the user's profile directory every time a window is opened" as is said in its description. Then I added the next code, found here into chrome/userChrome.js:

if (location == "chrome://messenger/content/messenger.xul") {
  setTimeout(function() {
    ZoomManager.setZoomForBrowser(getBrowser(), 1.2);
  }, 0);
}

})();

But it doesn't work, zoom isn't applied to the preview pane as when I run this command manually.
Maybe this code is wrong? I'm not that good in JS to fix it myself.