Try the Theme Font and Size Changer add-on.

http://chrisramsden.vfast.co.uk/9_Type_sizes.html

K4HM said

I see lots and lots of complaints but NO suitable answer! Most answers don't even answer the question but instead tell how to increase MESSAGE font size - not the main screen "message list". Suggestions to change screen resolution are common but are ludicrous. One would have to change resolution every time they went in or out of Thunderbird! CTRL+ is also suggested but does not work on the message list. Creating and inserting CSS into the profile is a ridiculous suggestion for most users. And changing advanced settings is only an option for a guru - from my experience less than 10% of the folks I know would even look at the advanced settings - and less than 10% of those few would actually attempt such changes. This has been an ongoing complaint for a looong time. It's time someone stepped up to the plate and provided a reasonable method of changing the size of the text on this screen. Or do you want to limit you users to only programmer types?

I have recently made the switch in the last week to Thunderbird from Apple Mail. I have been a Firefox user for many years. Near enough from the beginning. So far I am impressed with Thunderbird but it is lacking something that I don't get with Firefox. Font size is a big issue for me too and it's straining my already short sighted and possibly making (damaging) my eyesight more by using Thunderbird.

Though I agree with everything you are saying. I am one of those users who would prefer to change things within thunderbird itself rather than go and install another third party addon. I by no means am a guru but I have a fairly good understanding of things and would like to learn more about the about:config side of things treading carefully. I have tweaked things in Firefox this way. and disabled some items for security reasons. I am having difficulty finding info within the Mozilla site on this side of things and have yet to go down the search engine route for research into this.

I would like to change not only the main message list as you suggest but also the From, Subject, Reply To, To and Route content along with the Reply, Forward, Archive btns along with any other content etc, above the main message content. As well as my folders.

I feel Thunderbird is lacking accessibility features for those with bad vision or using smaller screen sizes. What I would like is a + and - with percentage btn like in Firefox that zooms in and out the complete page in a more responsive web like way. This would greatly improve things for many users. I'm sorry Mozilla but being able to zoom in and out with a keyboard shortcut in the main message content is not good enough.