There’s a sequence in Gaudium et Spes that addresses marriage and family, and is often quoted for its guidance on discerning family size. Children are the supreme good of marriage; marriage is ordained for children (though not solely so); educating the next generation; discernment of family size.
This is from the middle of that sequence, but it isn’t among the most well-known extracts. (Well anyway, they’re well-known to me from years in the natural family planning community. In any case, see #s 48 and the rest of 50 for that.) But I pick this bit because it’s a smaller, more-easily processed excerpt, and because I think it really crystallizes the big picture: we are cooperators with God when we bring children into the world, and we interpret that love through the way we raise our children. That’s big stuff!