Hold the needle with the cast-on stitches in your left hand and knit the stitches with the 13 circ. I think they just want you to cast on with the large needle so the bottom edge won't be too tight.
The ribbing is done on the 13s and then you go back to the 17--very common to knit ribbing on smaller needles--just knit to transfer the stitches then, too.
"Trust the pattern!"
Time flies like an arrow, but fruit flies like a banana.
Hop aboard while we follow the cloud-covered van of knitting harmony, man!! Woooo!! --Hedgehog
IngridKH on Ravelry