Depends on:
what your coop is like,
how many ducks you have,
if you feed/water inside your coop or not,
how long your ducks are in the coop,
your area/weather
I have a small coop inside a 10x10 dog run. My ducks ONLY sleep inside the actual coop. There's just pine shavings in there. Then the 10x10 dog run is there so they can be contained and safe if I'm unable to watch them or the weather is way too bad (aka raging river I don't want them to get into). There's straw on the ground of the dog run. They are messy no matter what you use. Mine do NOT have food/water inside the coop. They only have food/water in the run if necessary. Normally they are in yet another pen or free ranged during the day - depending on my schedule and the weather.