The gauge for the pattern is 3.1 st/ inch with yarn held double on size 10.5 (Pound of love is a worsted weight yarn).
The Baby Cloud is a super bulky weight that knits at 2 st/ inch without double stranding on size 15. I'd probably try a swatch single strand on size 13 and see if I liked the fabric, or got close to 3 stitches per inch, then either go up or down in needle size until I was satisfied.
I would DEFINITELY use circulars for this since the length is going to be over 2 feet. You could probably squeeze
all the stitches on a 14" straight needle, but I think it would be a struggle,
and not much fun as it got longer. Circs really help out the wrists on a large project like a blanket!