One consideration for the front especially, a skinny mudflap blocks much less spray than a wide one. Look down at your front wheel while on the bike, with your foot all the way forward -- you want your mudflap to block the line from your tire to your toes. I see many fenders with a decorative little flap that's perhaps twice the width of the fender. Those are good for keeping spray off your chainrings, which is a definite plus, but a much wider mudflap will keep more of the crud off of you, not just your bike.