small scale brewing questions

so I want to start brewing all grain on a very small scale, to experiment with making recipe's from scratch. (think 12 pk) I was wondering what the concerns associated with small scale brewing where, I know there is some concern over the head space to volume ratio in the fermenter, what problems does this create and why.

I was thinking of making my small system a steam infusion system out of one of the portable coolers. what sizes should I consider and why. thanks

You will lose as much as you bottle in a very small setup so it may not be worth the trouble. I'm looking into 3 gallon batches myself as a trade off between the work/reward ratio. 5 gallon for primary and 3 gallon for secondary. I can't really brew in my basement where I will have to ferment and store, etc, so I don't want to do 5 gallon batches at my age, running those things up and down the stairs.

Also consider you want enough to judge the effects of aging so a 12 pack might not be enough in that case.