I had been in the Hospital for over 5 weeks.. I had family taking care of my 2 babies and son. My 18yr old niece took it upon herself to bathe my Yorkie, Suzi, while I was ill . Problem now is that Suzi is full of Matts

I know my niece did not know, and did not ask permission to do this. NOW I need to clip Suzi.
I have clipped my Shih Tzu pup before and she turned out nice .However, with Suzi , and her hair being so much more different , I am scared I am going to hurt her.
I have a grooming table now , and clippers. Can anybody tell me, what size of blade I ought to use to take out her matts? Or is it just a matter of figuringt out?
My niece did not even know how to take her topknot out, so now Suzi has the top of her hair sticking straight up , matted . I am mad about all of this because her hair is med. long, now it is ruined.
With some guidance , I believe I can do this myself. And suggestions would be wonderful .