Advice on clothing sizes!

Linanne
So currently my daughter is wearing 3-6 months. She's 25 inches and weighs 14 pounds 11 ounces. I had summer clothes for her in sizes 6-9! But now I am thinking I am going to still need some more long sleeved shirts. I was at old navy and bought a couple long shirts but their sizes are 6-12 with weight 17-22 pounds. Any advice would be appreciated