We're using 6 qt tubs, though I think I need a couple of 15 qt ones as well for the bigger clutches. 12 eggs into a 6qt tub was...tight. I just need to find some 15's with clear lids so they are webcam friendly.
We measure temperature in the incubator (converted stand-up freezer), not in the tubs. Like was mentioned previously, as long as its well insulated that temperature should reflect the tub temps as well.