I would take a guage...
find the guage of the yarn over a 4 inch square
then....take your measurements...I would think between your head, so you can get it over your head, and your neck...so it won't choke you...
then multiply your gage x measurements=number of stitches to cast on.
so if your guage is 5 stitches to the inch, and your head is 20 inches and neck 14...I'd pick something like 18 inches....so 5*18=94, so cast on 94 stitches.
When knitting in the round as on circulars, just knit, don't do knit and purl to find out guage. So, cast on about 20 stitches, knit 1 row, rather than turning it around to purl, bring the yarn behind the work, and slide your knitting back down to the right side of the needle, and knit again, and keep going in this manner.
We knit a different guage without the purl, so to get an accurate guage, this is a bit better method.
I got this method for CO's on the www.socknitters.com website they give a generic sock pattern...I would think the method would be the same. Good luck!!!