I agree with weaver that people rarely play with browser size, so your menu is only going to be on two lines if people open the site on a small window which is rare.
You could do a few things to solve your situation.
Remove the padding on the wrapper ( at least left and right with the rule below in the wrapper BG CSS+ box
{padding-left:0px;padding-right:0px;}
Then you could reduce padding of menu items under a certain width with an @media rule like
@media only screen and (min-width:768px) and (max-width:800px) {
.wvrx-menu a {padding-left:0.5em;padding-right:0.5em;}
}
The default is 0.65em, depending if you implement the first suggestion, you will need to adjust the 800px value based on the padding value