You may be overthinking this.

Lay out all the stuff you need to take with you, leave half of it home and find the proper sized bag for the rest.

If you end up buying a lot of things in the US then you can get a bag here. I've seen tons of bags in various sizes at second hand stores that look near new for $5 or so but you can always buy a suitable bag at any mass marketer (Target, Walmart, KMart etc.) for under $20.