I do a lot of 2.5 gallon batches of experimental beers, or do so when I want to test a new recipe. I recommend 2.5 gallons because that is exactly 1 case of beer, and is exactly one half of any 5 gallon recipe in a book, magazine or website...so you can take any recipe and split it in half...you can do extract, extract with grains, partial mashes, and even all grain batches that way.
I give a turtorial on doing small all grain batches using an unmodified 2-gallon cooler inthe mr. beer thread..
all beer recipes are easily scaled so if you wanted to make a 10 gallon batch you just double everything ona 5 gallon recipe. So you can use that to scale down to .2 gallons if you wanted, not that you would