A block road network based on urban spatial dimension was constructed to address the problems caused by increasing closed units that restrain the urban economy of a city and limit public resources and to implement a block system. Subsequently, a suitable size of block opening was determined by conducting a network reliability analysis on the block network. From the perspective of network reliability, the limitations of traditional network entropy were analyzed in the research on block road network reliability. Maximum flow entropy and shortest path entropies based on network entropy theory were introduced from the perspectives of directed and undirected road networks, the calculation method of maximum flow entropy and shortest path entropy of the block road network was provided, and a reliability rating criterion for the road network was established. The maximum flow and shortest path entropies in the block opening process were calculated under different block opening phases. Then, the reliability of each road network was determined based on the reliability criterion of road network, which is used as the basis for determining the suitable size of the block opening. An empirical analysis was conducted based on a residential area named Zhongxuanliyuan in Shanghai. The residential area networks were established from the nodes classified based on residents’ life, leisure, travel, and others. The networks were divided into three stages based on the opening process, the shortest path entropy of each road network was calculated, and the suitable size of the block opening was analyzed and established. Results showed that the calculation of maximum flow and shortest path entropies can effectively determine the high-risk nodes of the block road network, and the network reliability has more advantages than the traditional network entropy. The network reliability determines the suitable size of the block opening based on the standard of block road network reliability, provides the basis for the phased block opening, and proposes new concepts and methods in conducting block system reconstruction.