Try and do an assessment of
all your likely costs and regular expenses for each potential system size. From that you can try to work out how much each pint/litre/gallon/keg of beer will cost you to brew, package and sell. Make sure you then leave a healthy margin on top for unexpected problems and things you have forgotten about.
Some market research - checking beer prices in the area, even just giving people some of your homebrew and asking them how much they'd be willing to pay for it, will give you an idea of what kind of prices you can expect to charge. Perhaps even take some (free) samples of your best homebrew round to some local bars and ask the owners/managers if they'd be willing to consider stocking your beer if you went pro. Wholesale prices will be significantly less than retail so you need to take that into consideration - but the advantage with selling to bars is that the location of your brewery is somewhat less crucial. That way you might be able to save a lot in rent.
Once you know how much your beer will cost you and how much you can charge for it - then you can work out how much you will need to brew in order to make a decent income. If you can brew that amount on a tiny system - then I say go for it!
I am in the planning stages for a micro/nanobrewery here in the Philippines. I have several advantages over the OP in that craft brewing has barely started yet in this country while interest in craft beer is taking off; labour costs are very low; land is cheap; and the amount of money I consider a good income will be much less than what most people in the US need (my current salary is only about $700 a month.) There are disadvantages too - high utility costs; cheap competition (though only mass-produced yellow lager; ) no local malt, yeast or hops so everything has to be imported; a lack of 2nd hand equipment that could be fashioned into something useable.
Looking at all my probable expenses and potential revenues I reckon I can just about make enough to live on with a 200Litre/batch system, (sorry I don't do gallons/barrels) brewing 3 times a week - if I sell my beer in bottles to local bars, resorts and restaurants.
But I am hoping to raise enough cash to afford a 500-700L system instead - even if I can only afford enough tanks to brew once a week initially. That way I can keep my day job, brew once a week and if my calculations are correct I'll make nearly double the money from my beer compared to my job! Then in a year or 2 - if my beer is popular enough - I can simply buy a few more fermenters and move to full-time brewing.
I have no interest whatsoever in running a bar or restaurant (though my wife might one day,) plus I can't afford the rents on decent commercial lots with high traffic, so this will be purely a distribution brewery.
Sorry this has turned into such a long post but at least its somewhat on topic
