How do you say my grandfather house is big in french?

There are a few ways to say "My grandfather's house is big" in French, depending on the level of formality you want:

Formal:

* La maison de mon grand-père est grande. (This is the most literal and formal translation.)

Informal:

* La maison de mon papi est grande. (Using "papi" for "grandfather" is more informal and affectionate.)

* Le chez-nous de mon grand-père est grand. (This translates literally to "My grandfather's home is big" and is a bit more colloquial.)

You can also use a more descriptive sentence:

* La maison de mon grand-père est très spacieuse. (This means "My grandfather's house is very spacious.")

The best choice will depend on the context and your relationship with the person you are talking to.