I got a Yakima Skybox, its like 81 inches long, never had a problem, it does have a weight limit, but if you put the heavy stuff in the middle its ok. its 6.7 feet long...I have had a 14.5 foot 76 pound sit on top Kayak up there...but the sky box is aerodymanic...
Length is only one measure. Total volume and weight are equally, if not more important. A ski box is quite different from a 20 cubic foot fiberglass monster.
First thing to check is the minimum cross bar spacing that the box allows. Read the installation instructions before buying.
Keep the Element's roof rack weight limit in mind. People do push the bounds that Honda gives, but it wouldn't be a good idea to go too far. Lots of weight up there will affect handling. Pay attention to the weight of the box itself. Don't the bigger ones weigh around 40lb?
Don't forget that you need to lift that to the top of the Element. Light weight is one of the reasons I like the smallest Thule box, the Sidekick. I can practically lift it one handed.