Site Assist will make use of CSS Menu Writer menus so you could create the menu as you would like it with CSS Menu Writer then use the preset to create a new site design that will use this menu.
Other than this you would need to identify the css that is responsible for the size in the css source files and make your adjustment there. There is a great tool for the FireFox browser that lets you take a look at the css that is behind the scenes and tells you where the settings are located. This FireFox extension is called FireBug and I use it all the time for this type of thing.