Regarding the weaknesses accounted for the centralized distribution state estimation (DSE), proposing decentralized formulations have been remarked in large distribution systems. However, predefined network division is usually assumed in the previous work and the effect of zone size (or the number of network zones) on the performance of the multi-area DSE procedures has not been investigated. In response, this paper presents a procedure to determine the optimum network division scheme for the decentralized execution of DSE in the active distribution networks. For this purpose, an algorithm for dividing the network into all possible zone sizes is proposed. As the first step, the network is divided into the maximum number of zones, and then neighboring zones are grouped step by step to increase the network zone sizes and obtain all network division scenarios. Finally, the hierarchically distributed DSE method is executed to estimate the network condition in all of these scenarios and the cost-effective scenario is recognized based on the newly introduced performance index composed of accuracy and runtime indices. The performance of the proposed method is tested on the revised generic 77-bus UK distribution network as an active network, and the results are presented and discussed.