I wanted to use all of the heavyweight plastic vitamin water bottles that our family was emptying at an astonishing rate and thought that they would provide sturdy sides for a shallow planter box. I could plant things in a shape formed by the bottle frame and plant in the bottles as well.
Lots of things I thought would work didn't and my first go around was not 100% satisfying, but I got it to work the next time around.
My box size was 9 bottles by 7 bottles (28 bottles total) and measured about 26 inches by 22 inches.
Note that I will not be growing edibles in this box. The combo of pre-treated wood, plastic and food.... not a good idea.