Width of a sub-menu

Hello to all,

I have created an item in the horizontal bar of the menu. Let’s say it’s called “Item”. The dropdown menu contains 3 items (let’s say Item1, Item2 and Item3).
Item1 is itself a dropdown menu containing 5 sub-items each with a title.
The problem is that the width of this sub-menu is quite small and the title of the items is bigger than the drop-down sub-menu.
How can I change this width?
Thanks in advance for any help,
Thierry