DK weight yarns are used for the 6-9 mos, 12-18 months, and 2 year sizes.  Worsted weight yarn is used for 4-6 year and 8-10 year sizes.  Finished chest measurements vary from 22.5 inches for the smallest size to 33.5 inches for the largest size.  Because these sweaters were first envisioned for a very small child, a neck opening was worked in the back to facilitate putting on the sweater.  If you desire to omit the back opening, work the back straight up to the desired length, binding off all stitches at once.