Hello
To amend try adding the following to the Custom CSS field of your templates parameters (Extensions -> Templates -> [YourTemplateStyle] -> Custom CSS)....
Edit to taste
.hornav > ul > li > ul,
.hornav > ul > li > ul > li > ul,
.hornav > ul > li > ul > li > ul > li > ul {
min-width: 300px;
}
Ciaran