6.5/7 would be fine, anything up to a 12 quart would be fine after that you're looking at it being too large for the snake (if you follow general advice).
One sec let me find Ralph's egg tub videos
YouTube - Pet Project 09 - part 3 ( clip 1 )
YouTube - Pet Project 09 - part 3 ( clip 2 )
LadyOhh's guide to setting up a tub:
http://www.ball-pythons.net/forums/s...ad.php?t=71770