I'd call that a 6 oz bottle, but I think it doesn't really matter as long as you know what you call it. The addition of the powder obviously does displace some of the water. But personally, I could call a bottle with 6 oz of water a 6 oz bottle. Sometimes if you make a smaller bottle, it doesn't quite fill it up to the next oz or ml marker, so then it gets confusing. But really, I don't think it matters as long as it makes sense to you.