I’m brewing one gallon of cider in a 2 gallon container with an airlock on top. Is there a problem with having all that space in the container?
You should be fine. When it ferments the CO2 will create an invisible protective lager on the surface. It will also push most of the unwanted air out of the airlock.
You may want to transfer to a 1 gal container after the majority of fermentation has taken place, but that is up to you.
