Natsu is an airy, summery, lightweight rectangular shawl. Knit lengthwise, it can be planned as long as one wants, then knit as wide as planned, or until the yarn runs out. The cast-on edge is long. It needs to be loose.
As many have stated before, just increasing the needle size doesn't help. The cast-on loops are larger, but that doesn't materially change the flexibility of the edge.
The yarn was found as a coup de coeur - Manos Serena, alpaca60%/pima cotton40%. The label says sportweight. I did not examine the yarn carefully. When I got home, I found that to be a lie. It is fingering, so I had to go back to the store to buy an insurance skein (or two).
I swatched the fabric. Then I cast on.
20 stitches of Twisted German Cast-on, when stretched til it screams, yielded 4" of length. Math - 158 stitches/20 = 8 x 4" =32" of wrap. Nope.
I tried a knitted cast-on, which is reputedly quite elastic. 20 stitches stretched equaled 6". Math - 158 stitches/20 = 8 x 6 = 48" of wrap. No sirree, bob.
So the interwebs brings me to the Chinese Waitress cast-on. Related to the knitting cast-on, the beleaguered knitter does not have to calculate tail length. 20 stitches = 8" stretched to its max. Math - 158 stitches/20 = 8 x 8" = 64". The pattern calls for a 68" finished length, and I would call that close enough, AND blockable to that endpoint.
K1 yo K1 yo is nice, mindless knitting. Until it isn't.