To enable userChrome.css in Thunderbird turn `toolkit.legacyUserProfileCustomizations.stylesheets` on in the Config Editor.

userChrome.css

/* Set default namespace to XUL */
@namespace
url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");

/* Set font size in folder pane */
#folderTree >treechildren::-moz-tree-cell-text {
    font-size: 12pt !important;
}

/* Set font size in thread pane */
#threadTree >treechildren::-moz-tree-cell-text {
    font-size: 13pt !important;
}

/* Set height for cells in folder pane */
#folderTree >treechildren::-moz-tree-row {
   height: 30px !important;
}

/* Set height for cells in thread pane */
#threadTree >treechildren::-moz-tree-row {
   height: 32px !important;
}