If you are making something for yourself, use any size your little heart desires. Why waste good fabric cutting things down if they fit together as they are? I read patterns the way I read recipes - to get ideas - then do my own thing. It's like that old story of the family pot roast recipe that started out with "cut off the end of the roast." Turns out somebody's gramma had a small pan and she always had to cut some off to make it fit.