By default, the dropdown menu in the primary navigation will wrap words onto multiple lines if they are too long, like this:

To prevent this from happening you can use the snippet of CSS provided below to set a minimum width on the dropdown menu, and set the items to “nowrap” which will keep them on one line, like this:

Paste the following CSS into your site’s child theme or wherever you add CSS to prevent the items in your submenu from wrapping:
.main-navigation ul ul {
width: auto;
min-width: 200px;
white-space: nowrap;
}