Megamenu columns size

Hello, is there any way for me to change the size of the megamenu columns? I’ve noticed their size is fixed, which means that if an item or heading is too large, it will “invade” the column on its right side.

As a matter of fact, in my particular situation, I don’t have this problem right now when I’m viewing the website fullsize:

However, if I change the window size:

Thanks!