Life-sized things are the original or actual size of to the thing (or person) they represent. A life-sized baby doll is exactly the same size as a real human baby.
Life-sized wax statues of your favorite boy band are just as tall as the singers are in real life — and just as disconcertingly smiley, though their wax skin might be a little shiny. A life-sized giraffe stuffed animal would be too big to fit through your front door, though you could bring home a dozen life-sized chipmunk stuffed animals with no problem. Life-sized, or life-size, dates from the nineteenth century.