You say "Chain/cast on a length to equal desired width", but how many stitches do I need to make a certain length?
The width of the Snuggle is up to you so it doesn't matter how many stitches you use as long as they are within the additional requirement of multiples indicated in the pattern.
For instance, if you want to make a Snuggle 20 inches wide, you would chain or cast on the number of stitches to achieve that width. For instance, when you crochet just measure the chain length for the desired width. For knitting, typically the number of stitches times 1.5 gives you your desired width.