Stack size dictates a player's approach to risk, aggression, and hand selection in a tournament. Deep stacks allow for more post-flop play, emphasizing patience and strategic maneuvering. Medium stacks require careful balance between aggression and survival, often leveraging position to accumulate chips without unnecessary risk. Short stacks demand urgency, favoring well-timed all-ins and fold equity to stay competitive. Adapting to stack size ensures optimal decision-making throughout different tournament stages.