Hi! IF you have the Rascal Dog Litter Box system (as we have) it should be big enough for a 9 month old Yorkie. I'm guessing since you said that your dog is no longer wanting to use it because she's "outgrown" it, that perhaps you have one of those flat, smaller box type by another company.
You might want to go to the
www.rascaldoglitterbox.com website to view the system. It is superior to ANY other dog litter box on the market. Our little Yorkie took to it right away and has NO problems using it and then going outside too (when weather permits).
I would additionally mention, you can email the owner of the company and she will help you with any suggestions, problems you might be having with regard to training. I believe she has tips right on the website. GREAT customer service and a pleasure doing business with!
Good Luck!