You just have to guess. I always start with 2 ounces and go up half an ounce from there. If they scream like crazy when you take the bottle out of their mouth, they probably need more.
Trial and error! I started with 2-3 oz bottles around 3 weeks. Sometimes she took it all, sometime she didn't. At about 3 months she would take 3 or 4. At about 5 months she started taking 5. Now she's up to 6 oz per bottle. I probably won't go up from there. I stuck to 6 with my DS until he went to whole milk at 12 months.
Susie, mom to DS 4/10/07 and DD 3/6/09 (MC 9/05, 2/06)
LOL - with both my boys I had such a crazy supply that if I were away for a few hours, I could fill an 8 oz. bottle to overflowing...from each side. Just needed to share that.
Re: Breastfeeding Moms - how do you know how much milk to put in a bottle?
I would do it, in the beginning, by how much I could pump per one boob for a session. So, for you baby's age, it was around 2-3 oz.
As they get older, they drink more, so it's a sliding scale. But try small amounts first.
Me with my littlest.