Your box spring needs to be the same length and width as your mattress size so that the bed sits perfectly on the box spring, with all edges securely on the bed frame. The height of the box spring depends solely on your comfort preferences. It might sound overly simple, but higher box springs are better for those who want to be elevated, while lower box springs are better for those who prefer feeling low to the ground. Changing the height of your box spring can change the height of your bed.