"The hallway is small."
Translation:Korytarz jest mały.
a usually long, narrow passage inside a building with doors that lead to rooms on the sides-
yes it can be przedpokój if your apartment building's designer was vicious,
It can be hard to distinguish between the two when we talk about office building or houses, but if we talk about "mieszkania" it is easy,
Przedpokój is a part of apartment, that has entrances to other rooms ( if it is not a room or a kitchen just a "transition area") korytarz is a part of the building with entrances to the apartments.