Here is what the pattern lists for all sizes:
FINISHED SIZE: Chest Circumference (allow 0 – 1" of ease). Sizes are divided by || symbol for ease of reading with smaller sizes in the first group and larger sizes in the second group.
6 mos (18") [9 mos (18.5"), 12 mos (19"), 18 mos (20"), 24 mos (21"), 2T (22)] || 3T (23") [4 (24"), 5 (25"), 6 (26"), 7 (27"), 8 (27.5"), 9 (28.5"), 10 (29")]
The tummy area won't matter as much since the flare begins right after the yoke ends. If your child's chest fits in the range of one of the smaller sizes, you can just customize the length to fit her.