I honestly wish it was flexible. 1 tank, 1 healer, 2 DPS seems fine as a small group. But since there are more DPS than anything else, it would be nice if when there are surplus of DPS, you could form a 1 tank, 1 healer, 4 DPS group and the dungeon scales up with more HP to offset the higher DPS (but do not do more damage to tanks or other things).

Of course we all know people would go with minimal size for loot, or maximum size for clears, since everyone min-max.