#_desktop_top_menu > ul[data-depth="0"] > li {position:relative;}
Añade eso a tu custom.css en el template de prestashop 1.7 o global.css en prestashop 1.6. Eso hará que el submenú (posicionado en absoluto) tome como referencia el posicionamiento del parent. Luego añade código CSS al child para maquetar el ancho. P.E.
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Add that line of code to your custom.css in your current template on PS 1.7, or global.css on PS 1.6. That will make your sub-menu (absolute positioned) take its parent's position as reference. Then just style your child to match your layout.
#_desktop_top_menu .top-menu[data-depth="0"] li .sub-menu {width:300px}
Please, since this is the intl. forum, it'd be appreciated if you'd compose your post in english 