。
My daughter weighed 8 pounds at birth.
Literal
Daughter [topic-は] was-born time 8 pounds [possessive-の] weight [copula-だった].
生まれた時 ('when born') uses past verb + 時 for 'when X happened.' Pounds (ポンド) is a rare unit in Japan (they use grams and kilograms), so this is almost certainly a translation of an English sentence. 重さ is the ~さ-noun of 重い ('heavy').