I remember doing a newborn top down sweater for my granddaughter. It was 20 inches around. The sweater length from the back of the neck to the hem was 10-11 inches (can't remember exact number but in that range). The sleeve length was 5 1/2 to 6, so that sounds about right. My pattern had me putting the sleeve stitches on hold and knitting the body of the sweater first. The extra increase is put at the sleeve bottom to fill in that gap and give the baby a little wiggle room. To check your work, measure across the back of the sweater from the bottom of the armhole to the other armhole across. Times this measurement by two. That will give you the circumference. Newborns have about a 20 inch chest. If you need to enlarge this sweater, you can keep knitting rows and doing the increases for the raglan sleeves until you reach this chest measurement, then follow the rest of the pattern. You'll have a few more stitches around, but it will fit. The bottom of the sleeve is the place to make these adjustments. Measure from the center back of the sweater to the hem to get the proper length according to size charts. Sounds like this one may be for a 3-6 month size. Don't worry about it. Babies grow quickly. The women in my neighborhood are giving birth to big moose babies that are 8-10 pounds. Must be something in the water. So you might need that bigger size. My biggest baby was 6 pounds even.
What I did was to look at baby sweater measurements for certain ages on Lion Brand's site. The simplest sweaters are just two rectangles sewn together and the sleeves are rectangles. This gives you a good idea of proportions. It doesn't have to be a knitting pattern. Crochet patterns are good for giving dimensions, too. If you scroll down toward the bottom of this pattern, it gives you some good baby sweater dimensions that you can use for other patterns. It will tell you if the proportions on the pattern you're using are correct or if you need to "tweak it" a little.