Even with the mods, this turned out newborn size. Part of the problem is my cast-on was too tight. I can’t get it over the baby’s big belly.

Mods for upsizing:

Gauge 4.5 st/inch

Cast on 80 on size 5 (very loosely!)
Rib for 2 inches
Switch to size 7
Knit for 4 inches (6 inches total)
Stop 2 stitches before marker
Cast off 4 stitches
(k2, p2) 8 times, k3
Cast off 4
(k2, p2) 8 times, k3

Do front and back as directed in pattern adding 4 rows to each.

I will try again with maybe these mods:

Cast on 84 on size 5 (very loosely!)
Rib for 2 inches
Switch to size 7
Knit for 4 inches (6 inches total)
Stop 1 stitch before marker
Cast off 2 stitches
(k2, p2) 9 times, k3
Cast off 2
(k2, p2) 9 times, k3

Do front and back as directed in pattern adding 2 rows to each, and 2 rows to the shoulder straps.