What went wrong?

Hello! I was trying to make the Lion Brand “Easy Triangle Shawl” with Homespun. The instructions say to use size 13 needles. They do not say circular needles. I used size 15 ‘cause I wanted a looser stitch. Finished width is 64 in.,length 35 in. How can this be done with 14’ straight needles? I finally had to stop when it was about 24 in x 12 in. because the needles were so full of stitches. What was I doing wrong?? Thanks for any help!

You did nothing wrong, but circular needles help hold the sts, a lot more than even 14" needles.