UPDATE - A few months ago I asked for advice about litterboxes.
Holly had started peeing up against the side and some of the urine was dripping on the outside of the box. I was wondering if I should get a covered box or one with high sides. After careful consideration of all the advice I got here, I decided to get two boxes with high sides from Petsmart online. When I went to pick them up, I was surprised at how big they were. All three of us could fit into one! But I took them home and wouldn't you know it? Holly never peed on the side of the litterbox again. Figures.
So the big high-sided litterboxes sat unused in my basement until today when I started thinking about how ridiculous it was that I had them and I wasn't using them. So I got one out and washed it off. I buy the 42-lb. package of Fresh Step that contains four 10.5 lbs. bags. The big litterbox has a line that is two inches high to indicate how much litter you should fill the box with. It took THREE bags to get to that line! That's 31.5 lbs. of litter!! In one box! Yikes! I would imagine that's a litterbox more suited for a tiger. I had some concerns about the opening being too high for them to jump in considering that they're seniors, so I washed out one of their regular litterboxes so they could use that if they couldn't get into the big one. After I washed it, I set it aside to dry off and about an hour later, I went downstairs to put litter in it and discovered that one or both of my cats had already used the big litterbox! So now they have their choices of sizes.
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Now I feel better about the big litterbox. As long as they can get in it, they will always have it! I plan to put out the other one one of these days.
some years ago. It's what my cats (all 8 of them) prefer. I still keep some of various sizes, in addition to the big ones. I have one room in the back of the house dedicated as a "cat bathroom"... all the litter boxes are laid out so each can choose the one they want, or at least one that is being used at the same time. It's cut down on a number of arguments back there, but oh... the joy of changing litter /snark. That is a LOT of litter to change, and a lot of boxes to wash down!
Still, we do what we can to keep our little masters satisfied, don't we?! 
As much happiness as we may give them, they give us back a hundredfold!