I looked up Ameribag as I was wondering what it was, I found this on their website:
Sizing Guide for the Healthy Back Bag® tote
Extra Small
15" x 8" x 5.5"
38 x 20.5 x 14 cm
Holds wallet, phone, comb, keys, sunglasses and water-bottle. Compact daily bag.
Small
17" x 9" x 6"
43 x 23 x 15 cm
Holds all daily essentials, plus diary, gloves, PDA and magazine. Our most popular everyday bag.
Medium
19" x 11" x 7"
48.5 x 28 x18 cm
If you normally carry a large, well-stocked bag, this is for you.
Also good for day-trips, sightseeing, etc.
Large
21" x 13" x 8"
53.5 x 33 x 20.5 cm
Very roomy. Great as an overnight bag, carry-on or good for day-trips and expeditions.
Extra Large
23" x 24" x 8.5"
58 x 36 22 cm
Mesh Bag only. Ideal for gym, dance, wet gear, and beach towels.
I've looked at these bags a lot, almost bought one similar (not sure if it was the same brand but it was the same shape/style) a few weeks ago, but didn't "need" it. so I passed.
Have fun!