to cool via air you need a large internal volume, but as you said, you also need a large enough volume to get radiators in too, from my research they're both need about the same size case and that is about 220mm wide.... height is important too, because if it's too short you wont get many PCI slots for Crossfire..... i.e the small Lian cases only have 4 slots .......... length isn't so important