Small batch homebrewing questions

What size batches are you contemplating? For a three gallon batch, I wouldn't hesitate to use a 5 gallon fermenter, or even a 6 gallon fermenter. If you're thinking about much smaller batches, then yeah, I would probably invest in a smaller carboy, although to be honest I'm not sure how much difference it would make. Easier to handle, though, which is a big plus (do you use plastic or glass carboys?).

 

The primary fermenter will be ok to leave it in because of fermentation, but I wouldn't recommend putting it in a secondary with that much extra head space for o2 to fill up.

 

Yeah, so, I'll echo @TooHopTooHandle's advice that for primary fermentations headspace is not an issue, but for secondary you want something without too much extra space.

That said, the common advice on this forum is that a "secondary fermentation" is pointless at best unless you are using it for a particular purpose (such as aging on oak, or aging a sour that requires months in the fermenter, etc.). For a normal, clean beer, most people here would advise you to pitch an appropriate amount of healthy yeast, maintain appropriate fermentation temperatures, and then package the beer once its gravity has stabilized, without a "secondary fermentation." (Personally I generally leave my beer in the primary fermenter for 2-3 weeks, even if it reaches terminal gravity within a few days. But that is not a universal practice and I would admit it's more force of habit than anything grounded in theory or personal experience.)

So in short, if you're brewing 2 to 3 gallon batches and you have no particular need to age the beer in a fermenter, I think you would do fine with a 5 or 6 gallon fermenter of your choice.