Can't be too long for a baby, it will grow to fit the garment. If it is long now, it can reach from the knees up and as the child grows, he or she will not change that much in the chest measurement but the length will. Some baby sweaters are 6 inches long after the armhole but you can sneak a tape measure into a store and measure a sweater in the baby department. I would guess that the measurement from the neck to waist would be at least 5 inches, don't want anything too tight. Looking at the picture of baby sweaters, they seem to be half and half, one half on top and one half on the bottom.
You can't make it too long. So here a couple options for you to consider. You are using metric measurements so you will have to do the math. Are you making up the pattern or do you have one you are using?
Carol J.