Originally Posted by old_townie
Is this going to be a secondary fermenter? If so you want Minimal Headspace... and for a five gallon batch, even a six gallon would be not enough!
that's sort of confusing the way you wrote it. If you want minimal headspace to condition a five gallon batch, use a five gallon carboy, not a six or larger.
Having said that, my opinion differs. If I had it all to do over again, I'd buy only 6.5 gallon carboys for those 5 to 5.5 gallon batches and use them for fermentation and for conditioning. The advantages are that you have the greatest amount of space for kraeusen during fermentation, and when you siphon to "secondary", you can condition a 5.25 to 5.5 gallon batch easily without running out of room.
Running out of room in my secondary happened to me many times because I normally shoot for 5.5 gallon batches and I am a very careful and thorough siphoner. I hate wasting a pint to a quart of beer because it won't fit in secondary, or having to find a half gallon jug for it to condition in, especially if I'm dry hopping.
I personally think that the extra head space during the conditioning phase is irrelevant. There is still some yeast in suspension. It continues, albeit very slowly, to ferment and produce a very small amount of CO2. This will fill up the head space, push out the oxygen, and you wont' oxidize your beer. If you want, you can give your conditioning carboy a shot of CO2 from your kegging gas before you stopper it up, and then you have no concern at all.
It is really nice having all the same size carboys and not having to worry about whether a certain size is available at any given time.