Now that you know how large of a litter box to get, the type of box is just as important. Get an UNCOVERED litter box. If your cat tends to urinate over the edge, you can get high sides such as this one.
Covered litter boxes are made for people - so we don't have to have the litter visible, but cats in the wild do not go into holes or caves to use the restroom; they are vulnerable and can not see predators coming.
Cats prefer litter boxes to be open at the top, one foot away from the wall and have at least two escape routes - which means don't back the box up into a corner.