I tried reproducing what you have in the screenshot, but I am getting all the words perfectly in one single line, no matter how many words I add. but still if you need to set a width to it then put the following code into custom css in theme options and see whether it helps.
#menu-item-5061 {
width: 130px;
}
If you can send a link to your site, then I can debug better. Thanks!
It is wired because I also tried to delete the page and create a new one, but it also breaks the line!
Your suggestion did not work 100% for me. But when I added your suggestion to the CSS and changed float from left to none in #primary-menu > ul.menu > li > a it worked.
Anyways I will post the URL when it is online for you to see.