SHANGHAI — Ai Weiwei, a dissident artist who was released last month after spending 81 days in a Beijing jail on tax evasion charges, spent his days of confinement in a tiny room with only a bed for furniture and two police officers who monitored his every movement, even standing next to him in the shower, his sister said Thursday. For exercise, Ai paced in the small room, only six floor tiles long, but he estimates he walked the equivalent of 600 miles and lost about 30 pounds.