Zip Up The Back Sweater comments:

It is your knitting gauge rather than the size of yarn you use that determines size. I use the knitting standards sizing charts. So I ran a few numbers with this pattern, which is almost my favorite. I make a few changes in it.

http://www.knittingonthenet.com/patterns/babyswezipback.ht

As written, it is a size 12 month size sweater, that is 6 st. and 7.5 rows per inch.

At 5.5 stitches per inch, it is a size 18 mo.
At 5 stitches per inch, it is a size 24 months.
Primary difference between a 24 month and a size 2 is in length.

Now for the lengths use ruler measurements rather than the number of rows.

length to underarm, (underarm to waist) for a size 12 mo. is 3.5 and (3.5)
for size 18 mo is 4 and (4)
for size 2 is 4.5 and (4) Ahhhh.... go ahead, make it a 4.5

On the average another 2 inches is added to the length of a sweater for below the waist and then ribbing is added.

So now for your yarn, baby yarn will make a delicate light weight sweater, sport weight will make an average weight sweater with a soft hand. Worsted weight will make a tighter knit, warmer sweater. Choose whatever one you want and then work on your correct stitches per inch.

Note: difference between a 18 mo and a size 2 is 2 inches around the chest and 1 inch in back waist length. 3 inches in sleeve length.

Changes I make to pattern: knit until center front piece is 10.5 inches across. This will lower armholes about 1/2 inch. Measure for sleeve from center back down to cuff. Make it 17 inches and then add a 2 to 3 inch cuff. This will allow you to fold up the cuff at first and then lower it as the child grows. Benefit--another year of wear. This is the size being worn in the photo of my great grandbaby.

Other changes: I use a 2 stitch garter band on both sides of the zipper. Stitch the zipper in by hand with your stitches at the back of the 2 stitch band. Use a back stitch or picking method. This is the exact amount needed to cover the metal part of the zipper and help eliminate the yarn catching in the zipper. I also put a stitch between the increases along the raglan lines. That's just because I like it that way. Please note: the sweater in the picture is sewn together incorrectly. Match the sides of the sweater on both sides of the zipper. One side should not be longer than the other.

How do I know this stuff? You start with 3 children, then you add 12 grandchildren and follow up with 5 great grandchildren. I've knit more sweaters than I can count.