Size of Demijon

You won't want one much bigger than that. Once your wine has passed the bulk of its fermentation, it needs to be kept in a container with as little headspace as possible, to avoid exposure to air.

Of course, if you have say .. a 5-gallon demijon, just make 5 gallons of wine! You'll be glad you did.

-- WB

The answer is basically that you need a bigger one. How much bigger is not so easy to answer. If you use your carboy as a primary, I prefer a bucket, then you only want it 2/3's full. During secondary, you will want it filled much more. Maybe half way up the shoulder. When you bulk age, you want it filled up into the neck, within 1 inch of the airlock. As you can see, you cannot use one size for a batch or you will need to do some kind of adjusting. I like to keep an assortment of sizes so I can use what is best. I use 12, 6.5, 6, 5, 3, 2.8, 1, and 1/2 gal sizes and have many of each. But then I have been collecting them for 30 years and my insanity is still growing. If yours is just taking wing, collect as many 1 gal jugs as you can get your hands on (you can always go to several smaller jugs) and pick up the larger ones as you can.

Ray