my mother, who has five children, gave me some bottle advice the other day, because i was shopping around for them.
she told me that the 4 oz. is best for newborn babies, who are only a couple of weeks old. once they get a bit older, move them up to an 8 oz. or larger bottle. we also discussed glass verses plastic. nowadays, plastic are more common, and it can be quite hard to find the glass bottles in stores. my mother said glass bottles worked best with us because they stored the heat longer, they're also easier to wash clean. just don't leave your baby alone with one, because it could break.
i purchased glass bottles - both 4 oz. & 8 oz. - online. they were really inexpensive. i've also purchased a couple of plastic ones. i'm staying clear of drop-in's because i've heard a lot more negative responses than positive. they do make a lot of different bottles now, and most of them swear "air bubble free." i honestly think it's just personal pref. most of the time it's not the mother who really chooses which bottle they like best, but the baby does. i know a lot of mothers who had to try a variety of bottles - different shapes, different brands, etc. - before they found one their baby liked most.
good luck!