You can use any size for any course. However, below is how each size is most typically used. 

PLATES:
Large Plate – Entrée
Medium Plate – Salad and/or Dessert
Small Plate –  Bread Plate or Amuse Bouche. 

FLATWARE:
Large Knife/Fork/Spoon – Entrée Course
Medium Fork – Salad, Dessert and/or Appetizer
Medium Spoon – Dessert and/or Appetizer