These days it seems that anything and everything goes. I was looking at the scarf section at work one night and it seemed that every possible combination was there. Long or short, skinny or wide, even a couple types of keyhole scarves as well as various cowls.
I would take into account how you think the person might wear it. If your climate is mild, a more decorative scarf is in order. If you have a cold climate, something that offers a little more protection from the elements is a good choice. Also, how much of the yarn do you have? That will also dictate how many options you have.