Text (6th/7th/8th/9th): */*. ; ~this~'s power and toughness are each equal to the number of cards in your hand.
Can be sacrificed to a spell that is being cast, and which would leave your hand empty. Your hand is not actually empty until the announcement is done and by that time Maro has been sacrificed. [Duelist Magazine #18, Page 28]
If Infernal Harvest is the last card in your hand and it is announced, Maro doesn't die. This is because the Swamps return to your hand as a cost and are in your hand before state-based effects are checked. [Jordan 2001/12/16]
His toughness is checked as a state-based effect. This happens right before a player gains priority to play a spell or ability. If a spell had your number of cards go to zero during the middle of resolution, but you did not have zero when the spell ended, then Maro would live. [D'Angelo 1999/05/01]