My runner eggs are about the size of a jumbo chicken egg, though sometimes they are the size of a large or extra large (if they have just started up laying).
E.T.A. I don't find them so messy. But I have experimented with several ways of keeping up with them and it has worked out. They certainly are not no-maintenance creatures! I just rotate straw and bedding through their housing and then onto the compost pile or garden beds.
Regarding water, a good splash catcher avoids much mess. One of the every day or two cleanup chores that takes five minutes is scooping wet sawdust out of their splash catcher and onto the garden path, replacing it with a few handfuls of dry sawdust. Two more minutes to pick up damp bedding at the entry way to the catcher. Since it is part of the routine, it doesn't seem like much to do, and it keeps their area from being messy.
I'd say if you know what to expect from their behavior, you can design a setup that helps you keep everything in pretty good shape (it won't be sterile (c; ) without taking much of your time. We do not have odor problems, or fly problems, or mold problems. Time to go play with the ducks!