When cast, Zook's Doubling Aura doubles the effects of of any spell level 2x less than the spell slot used. For example, if you cast Zook's Doubling Aura on the 2nd level Enlarge Spell,  you grow 4x the size instead of 2x, but you need to use a 4th level spell slot. On spells such as acid arrow, where you summon an arrow of acid from your hand, it does not double the damage. Zook's Doubling Aura creates two arrows instead of one. However, you can launch both at the same area, inflicting double the possible damage of the original spell.