To change the height of the menu bar, you’ll want to increase the Vertical Padding of the top level menu items. This can be done in the Customizer > Top Level Vertical Padding.
The total height of the menu bar (really individual items) is the line height (which is affected by the font size) + padding + menu bar border. Generally you don’t want to alter the actual ‘height’ property directly, as this will make the layout more brittle. To keep things as flexible as possible (which is very important for responsive sites), adjust the padding or font size as necessary to achieve your desired menu bar height.
Another way to change the menu bar height since version 3.3 is to use the Top Level Line Height setting, provided that your menu item text does not wrap onto multiple lines.