You can figure out the width the pattern intends by taking the number of sts and dividing by the sts per inch. Most are 5-8" wide with 6" being about average. The length can vary, it depends if you want a shorter one to just cross over the chest (about 48-55"), one to wrap around the neck (60-72"), or a super long one to double up and pull the ends through the loop (72-90"). Usually something about the same as the height of the wearer is good.
sue- knitting heretic