does size matter??

My kitten was sick and she stopped using her litter box, now it seems she sometimes uses the box and sometimes doesnt even tho she is no longer sick (she always pooped on the rug when she was sick). I stopped using lines. Could it be the size of the litter box? She is 5 months old and our litter box says for medium sized kittens. Is that the size for her age group, she weights 5lbs. Im desperate to find out why she is still doing this.

Try adding a second litter box, most cats prefer to poop in one and pee in the other.