The size you're making is determined by how many stitches you're using.
So work however many stitches are you are using for your size, then use a second ball of yarn and bind off the center X number of stitches.
Now you will be working the two shoulders with each having it's own strand.
The neck edge, naturally, are the two inside edges. You will bind off 2 stitches when you are at the beginning of the next edge one time each, then decrease on ever other row (when the neck edge is the first stitches on the needle) twice. Then work without decreasing. . .
"Trust the pattern!"
Time flies like an arrow, but fruit flies like a banana.
Hop aboard while we follow the cloud-covered van of knitting harmony, man!! Woooo!! --Hedgehog
IngridKH on Ravelry