Variation in Navigation Height (Flexible navigation bar block region height)

Hi there. Using the Zen theme for Drupal 7, the height of my "nav bar" changes depending on content but it should be set to a fixed height in layout-fixed.css or layout-liquid.css. The only solution I have come up with so far is to create another theme and using the themekey module have it show a different theme for specific content. As this solution seems messy and complicated does anyone know of better, easier solutions? Thanks!