The jelly roll strips come from yardage, which can range from 40" to 44" for width of fabric, plus you have to allow for seam allowances, so there's some room for variance there.
To me, the crucial factor is that you start with a strip of 2.5" (2" finished -- we'll go with finished), then fold and double to 4", then fold and double to 8", then 16", then 32", then 64" -- and if you could fold again, you'd be at 128". Like Cad Queen said.
So I can see making a small blanket 32" wide by whatever depth, depending on how many strips, or go for 64" and whatever depth, but I don't know how people come up with a blanket 42" or any other width.
If you have 22 jelly rolls, that would be approximately 900 inches. I write in two columns, with the left at 2" and the right at 900. Next line down, 2" becomes 4", 900 becomes 450 (cut in half). Next line, 8" and 225", then 16" and 113", and then 32" by 56". If those dimensions work for you, you could add a border and call it good. If it's a little too tall for the width, you could drop a jelly roll or two...
I hope this makes sense. I find I have more fun thinking about this pattern than making it. I tried it once and threw in a couple of half strips to try to create variety and ended up with a bit of a mess...but that's what I get for not following the directions!