There are a couple of ways of updating the width depending on what tools you have access to. If you have Menu Writer installed you should be able to open the template or the home page and select the script icon for the menu to bring up the edit menu options for Menu Writer. If you do it this way you can adjust the width of the menu directly from the edit theme or edit menu option, the width is defined in the box tab.
The other way to do this is to make the edit directly in the css. Inside of the CSSMenuWriter folder you will find the cssmw_content and the cssmw_home locations. In each of these there is a menu.css file. This is where the width is defined. If you just do a search in this css file for 'width' you will find where it is specified. Once you update the style here you can also copy it into the siteassistpro_css > mystyles.css file to ensure that the update is retained if you make an edit to the site.
The width of the navigation is determined by the width value times the number of links you have in the menu. To have it fit more links into a single line you will need to reduce the width so that it can accommodate all of them.