I like 1/3 size batches of 1.67 gallons, so just divide all the ingredients in a standard recipe by 3.
I've gone as low as 1.25 gallons (which would be dividing by 4) but 1 gallon (divided by 5) just seems so tiny to me, you only really get a 6-pack out of it after trub losses. I still do small batches like that for cider and mead to fit 1-gallon containers to limit head space, but for beer that won't be sitting in a fermenter for many months, it just doesn't seem like 1 gallon is enough. But to each his own. Just some stuff to consider.