The flexibility of the ASP.NET AJAX dropdown's item template support will serve you well when you need a multi-column dropdown, or any sort of sophisticated presentation and layout of list item contents within its dropdown window.
No matter in what position you place the dropdown on your Web form, its intelligent auto-positioning will open its contents in the direction that ensures you of the easiest user experience.
When you have a large number of dropdown items, you can display a pager so that the user can flip through their list items, loading them one page at a time.