I'm looking into buying a decanter and am finding a variety of sizes from 24oz to 40oz to 70-80oz. Which size is most appropriate for decanting 750ml bottles?
I presume that when the full bottle is poured into the decanter, you want the level of wine to come to the mid point of the widest part of the decanter as is shown in this image to maximize air contact:
Is this correct? If so, when I fill a 24oz decanter, how "full" is the decanter? Should I aim for a 24oz decanter (and expect a 750ml bottle to fill it to the mid-point of the neck), or will a 750ml bottle only fill it to the point indicated in the image? Or should I aim for a 40oz decanter to help maximize surface area exposure to air?
If you are needing maximum aeration, the type you have shown is correct. Most are either single or double bottle (magnum). Typically, they fill to the widest part, as you say.
If you are simply wanting to carefully pour (or filter) the wine off the sediment, but not particularly wanting to expose the wine to a lot of air, then you would want a decanter that is approximately the shape of a wine bottle -- a lower section cylindrical or square, with a neck, and holding approximately one bottle (or two for a magnum - rare) with the fill level well up in the neck. A fifth and a .750 Liter bottle are almost the same - about 25.6 oz and 25.35 oz respectively. A magnum of course is twice that. If your bottle of port is slightly too much for a 24 oz decanter, well, you need to have a sip right away to see what it tastes like pop-n-pour.
The picture is that of a ship's decanter. Not only does it maximize surface area with air, but the wide base prevents movement on a rocking ship. For an old, fragile wine, you might prefer a decanter with a smaller surface area.
Ok, I think I'm beginning to understand... Is it better, then, to use a ship's decanter (lots of surface area exposure) for the younger wines that need to be decanted for longer periods of time and use a more narrow decanter for the older wines that don't need to be decanted for as long?
You could look at it that way. I think, however, the physical pouring of the wine actually aerates more than having a large surface area; but when you want lots of air, everything helps. Some people will double decant very young wines to give them lots of air; I will sometimes also, but you can overdo it as well.
