In my installation of Puppy Linux 4.3.1, you need to travel to /root/.thunderbird/dhget5jr.default/chrome. (The "dhget5jr" is a variable that will be different on your particular system.) My .default folder did not have a sub-folder named "chrome", so I created it. Then in my text editor, I created a new file named userChrome.css, and populated it with the following:

/*
* Do not remove the @namespace line -- it's required for correct functioning
*/
@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* set default namespace to XUL */

/* Global UI font */
* { font-size: 12pt !important;
font-family: Verdana !important;
}

Restart Thunderbird, and voila! bigger fonts everywhere. (Your eyes may be younger than mine, so try substituting 11pt instead.)

I hope that this helps.