Funny you should ask this just now. My DD and I just measured three totes we like so that I can make her one. We decided on 15" x 15" on the top and 10" wide bottom She needs longer straps (she's almost 6'), one hand-sized pocket on the front and a water bottle pocket on either end. I will have to look for a pattern to follow too.
If you go to ribbon above to SEARCH and type in tote or tote bag it should bring up lots of info. Good luck.