Now that's an interesting topic!
Here's what I can say for sure: when you consider a half bottle, as in half full, you will use "une demie bouteille".Now if you're talking about those smaller bottles you can buy in shops, we don't really have a specific word for them, other than "une petite bouteille de vin" or again "une demie bouteille de vin".You can refer to "un demi" when you order what is equivalent to half a pint [more for beer though] or about 25cl / a quarter of a litre.
Voilà ! I hope that answers your question!
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