Menu Limit to the size of the page

I am using the Vantage theme to create my company’s site. In my products dropdown menu (classic menu of wordpress) i have put aroun 15 product categories. The problem is that not all the product categories show as the reach the down of my page. Any ideas how i can put a ”scroll down” feautre to the menu if there is not enough space in the page? Or any other solution?