Choose one or more— • Destroy all small creatures. • Destroy all medium creatures. • Destroy all large creatures. (A small creature has total power and toughness 4 or less, medium is 5-8, and large is 9 or more.)
If you choose more than one mode, creatures are destroyed in the order specified. This may cause some creatures to escape destruction. For example, if you choose to destroy small and medium creatures while you control a 2/2 creature with the ability "Other creatures you control get +1/+1" and another 2/2 creature that's a 3/3 creature thanks to that ability, the first creature is destroyed, then the second creature won't be destroyed when medium creatures are destroyed since it's become a 2/2 creature.
If a creature has negative power or toughness, consider the actual value when determining the size of the creature. For example, a 0/7 creature that's been given -2/-1 is a -2/6 creature, which totals 4, making it small.