I added a crochet collar to the sleep sack and I am now satisfied that it will be a nice 6-12 month size. A friend who is an experienced knitter was kind enough to re-write the pattern, and make a test sack and this one is much more appropriate for the 0-6 month size. I will be casting on with 88 (not 128) stitches and increasing gradually after the waistband to about 130 stitches. I'm also going to taper the bottom a little for a more streamlined look. I'm excited to see how it will turn out!