Knitting with Two Strands and Size Question


I will be knitting a Snuggle to donate to an animal shelter via the Snuggles Project ( For the Snuggle I want to make, I will need to knit with two strands of yarn. Do I cast on with two strands or yarn or do I only start knitting with two strands of yarn in the first row after the cast on?

Also, I am not told how many stitches to cast on. I am directed to cast on as many stitches as I will need for the particular size Snuggle I want to knit.

The sizes are:

14" x 14" for cats and small animals

24" x 24" for cats and small to medium dogs

36" x 36" for medium to large dogs

How would I determine how many stitches to cast on for the different sizes?

Thank you,