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Saving power in datacenter networks has become a pressing issue. While in operation, ElasticTree and CARPO can save power consumed by a fat-tree network by using sleep mode where some components such as ports and switches are turned off when traffic demand in the network is relatively moderate. In this paper, we propose a new approach by exploring the design stage of a datacenter network and focus on how to choose the right switch size that can potentially save the most power during the expected operation of the network. We also consider speed scaling where the power of a switch can be varied by adjusting its processing rate according to its traffic demand. We use analysis and simulation to investigate the power-saving performance of different switch sizes, power-saving modes and traffic demand patterns. Our findings with sleep mode reveal that deploying a large number of small switches is more power-efficient than a small number of large switches when the traffic demand is relatively moderate or when servers exchanging traffic are in close proximity. With speed scaling, the reverse is generally true.