There are some stylish flat fenders out there (e.g. these). Some, like the one in the preceding link, seem to only be as wide as the tire.
This question is posed to people who have used flat fenders:
Do you ever find that you get some splash, say on your legs, or the outer edges of your butt/back? Or are they completely fine?
When answering, please also tell me whether your flat fender was the same width as the tire (as in the link above), or a bit wider.