Measure your neck as suggested then multiply that times your sts/inch; I think you want about 13-15" for the neckline, maybe a little more. Place markers - 1/6 of the sts for each front and sleeves, and 1/3 for the back. Increase before and after each marker, try it on after about 9 inches and see if it's big enough, if not, keep going.
Ahh, here's a pattern for it in DK weight, not bulky. If your yarn is worsted, use a CO number in between the 2.
sue- knitting heretic