[DoDe]

You may find the font inside the styelvar of your style under the: navbar_selected_popup_body_a_Color
As for the height you may find it under the: navbar_tab_size but if I were you I would play with the template vbulletin-chrome.css (of course first make a backup).
Inside that template you have this code

PHP Code:

.navtabs li a {
    height:{vb:stylevar navbar_tab_size.height}px;
    line-height:{vb:stylevar navbar_tab_size.height}px;
    padding:0 {vb:stylevar padding}; 


Play with it otherwise you may find that changing the height in the stylevar the entire navigation bar will change proportionally.