The ending you put on the adjective depends on whether the noun is singular or plural, and whether the noun is common gender or neuter.
Use no ending in the case of a singular common noun:
Bilen er stor.
Use -t in the case of a singular neuter noun:
Huset er stort.
Use -e in the case of a plural common or neuter noun:
Bilene er store.
Husene er store.