I would go as big as possible...6x8 min....it's not just the (first) 6 birds......
It's the storage of feed and supplies right there where you need them,
which is nice to have separated from the birds area.
It's the additional birds you will want in the next year or so.....it takes extra room to integrate new birds.
Yet again nice to have a separate area for new chicks, sick bird, broody bird, bad bird.
A temporary wire wall can work wonders...I designed one into my coop, with a separate people door, best decision I made. Check out My Coop page too.
You can probably whip out a coop real quick with your skills,
but you might be happier in the long run if you take some time to plan for the future.