The shallow depression in my run measures about 8x8 and is not more than 2' deep - still deep enough for a randy drake to grab a hen by the back of the neck and dunk her when she tries to ignore his attentions. A liner 15x15 would easily address that, and with a little adjustment, I could probably do 20x12 or similar. The idea would be to get about 100 sq ft of surface area for 8 birds. For 6, I'd be trying to get about 75 sq ft +/-. That's between 8 and 9' square, plus material to go up the sides and be covered at ground level. 12 x 12?