YOU SAID:

I have a small dog who thinks he is a large dog

INTO JAPANESE

私は彼が大きな犬だと思っている小さな犬を飼っています

BACK INTO ENGLISH

I have a small dog who thinks he is a big dog

INTO JAPANESE

私は彼を大きな犬だと思っている小さな犬を飼っています

BACK INTO ENGLISH

I have a small dog who thinks he is a big dog