Central business district (CBD) is the crucial area in modern metropolitan areas. In this study, an evaluation system is proposed to help transportation professionals determine the reasonable scale of road network in CBD. The evaluation system consists of two parts, including a indicator system and a selection system. The indicator system is based on the measurement of several parameters such as the regional road traffic capacity, smoothness of regional road network, unit road load, and regional road network structure, etc. The selection system uses the maximum entropy with the results of the indicator system to select the reasonable road network scale. A case study is also conducted to apply the whole evaluation system to Huaqiao central business district. The research results show that the evaluation system developed in this study can be used as a useful guidance for determining a reasonable road network scale for the city central business district in metropolitan areas in China.