She pretty much needs a 6 month size for width and an 18 - 24 month for height. (nourishinglittlesouls.com)
Most babies won't fit newborn for very long at all, so consider stocking up on 0 - 3 month size clothing and keep the newborn sized items to a minimum. (themonarchmommy.com)
They are soft and the 18 month size fits her well at 22 lbs. (pottypatty.com)