I had a bag, once. It was the perfect size to fit my sketchbooks, pencils, a water bottle, and a few books to keep the kiddos entertained. Alas, it wore out. A new art satchel was needed, this time with a little more flair. I kept the dimensions the same but added some stripes--because really, everything is better with stripes.
Medium Satchel/Tote Bag
(18) 2½” x width of fabric strips
1 1/3 yards heavy fusable interfacing
½ yard lining fabric
16” x 35” batting