Hi there!

You can adjust the menu item font and/or padding value among them with a media query to get them fit in one line.

Here is a sample code for you you need to put in your custom CSS:

@media screen and (max-width: 1371px) and (min-width: 991px) {
.sp-megamenu-parent>li>a, .sp-megamenu-parent>li>span {
   font-size: 9px;
   padding: 0 5px;
}
}

You can modify the values as per your need!

Note: Path towards custom CSS: Extensions>Templates>Styles> Click on your current default template name then choose Template Options. Then find custom CSS within the custom code.

Best Regards