Tú casa es grande.
The difference between "tú" and "usted" depends on the formality. "Tú" is used when talking to friends or relatives (informal) while "usted" is used when talking to older people or a person of high social rank (formal).
In this case we want to write the informal form of "tú", so we use "tú". Also, when dealing with permanent states, the verb "ser" (to be) is used.