What is one quote in the play that directly states the large area of the house before the existence of the city?
I believe the passage you're looking for comes from the opening pages of the play.
LINDA: We should've bought the land next door.
WILLY: The street is lines with care. There's not a breath of fresh air in the neighborhood. The grass don't grown any more, you can't raise a carrot in the back yard. They should've had a law against apartment houses. Remember those two beautiful elm trees out there? When I and Biff hung the swing between them?
LINDA: Yeah, like being a million miles from the city.
WILLY: They should've arrested the builder for cutting those down. They massacred the neighborhood. More and more I think of those days, Linda. This time of year it was lilac and wisteria. And the peonies would come out, and the daffodils. What fragrance in this room!
If compare this description of the house that is provided in the opening stage directions and also to the description provided late in Act II, we just how dramatically the Loman house has changed over the course of these past 25 years.