Ciao.
By default you cannot, I have seen Menu module that allowed to have that, but using extra Menu - too complicated, and it was paid extension.
From what I also used in the past was using max-width value for menu item class
.sp-megamenu-parent > li > a, .sp-megamenu-parent > li > span { }
this is easy.
You can also using extra class name for menu item to use bigger or smaller max-width value. Of couse line-height must be also controlled.
By now, I don't have other ideas. Maybe find website that use that effect and check how they did it.