Here's a final summary on how tote bags are displayed, meaning which size bag is shown first.
On your main image page where you have one piece of art, the largest size is displayed first.
On your personal "Shop" page on the Pixels website, the medium size is displayed first.
On the Pixels Main "Shop" page, for all artists, the smallest size is displayed first.
My sleuthing is over for the day. Hope this helps someone else besides me :)