I dunno...I'm a firm believer that "size doesn't matter" so what about if you found out what style their bedroom is currently in, and make some glorious pillow shams? You could take your time and pour your energy into a smaller project that would pack a big punch, kwim?
Of course, how well you know the bride-and-groom to be and your relationship to them DOES kinda play an important role. You want the gift to be from your heart to theirs, whatever you decide.