The box will always take on the width of the longest post title since the dropdown does not have a set width and does not wrap the titles. On some themes, there is an "overflow: hidden;" set on the sidebars to keep stuff in the sidebar from running over onto the content area, but apparently not with Selectra. With overflow hidden though, it will cut off the right end of the pulldown. It will still work, but it will just look a little funny cut off on the left side.
My suggestion would be to put it in a page instead accessible from your top navigation and then you would not even have to use the dropdown, just put it as a straight list.