One of the most important issues in urban development is the optimal size of the city and acceptable scale for its development. The importance of this issue is to the extent that many other urban issues such as the urban hierarchy of access, utilities distribution and etc. are influenced by this issue and so, defining the optimal size of the city, is very important. Till now, some research have been done in this regard but innovation of this research is that has responded to the problem of "the optimal size of the city" based on Islamic view and in particular juridical view. In this way, by citing the juridical teachings of Shiite, First, the favorable geometric system for the Islamic city and the center of this system, is inferred and then, by citing the juridical rulings of the jameh mosque (as the geometric center of the city), minimum and maximum acceptable size for the city is concluded. So, the main axis in the process of data analysis in this article is the juridical rules and laws of prayer on the day of the congregation (Friday) and consequently, the juridical rules and laws of jameh mosque. The most important religious document in this regard is the chapter Jumue in the holy Quran in particular, its final verses which are cited as follow: Believers, when you are called to prayer on the day of the congregation (Friday), hasten to the remembrance of Allah and put your trading to one side. That is the best for you, if you but knew (9). Then, when the prayer has ended, disperse in the land and seek the favor of Allah, and, remember Allah often, so that you prosper (10). Yet when they see some commerce or amusement they flock to it, leaving you standing, say: 'that which is with Allah is better than the amusement and commerce. Allah is the best of providers.' (Chapter Jumue, verses 9-11). According to these verses, attending the Friday prayer is very important and is a duty of every Muslim. On the other hand, this prayer venue is jameh mosque while, there are important juridical laws for this kind of mosque in Shiite jurisprudence that some of them is related to the distance between jameh mosques. According to this and regarding to this issue that each city has just one jameh mosque, the optimal size of the Islamic city is deduced. The research method which is used in this article is a compound method (combination of qualitative content analysis and logical-argumentative research). In this way, first, propositions (especially the juridical hadiths) that directly or indirectly are related to the problem of this research have been extensively investigated using qualitative content analysis method and then, using the resultant of propositions analysis by logical-argumentative method, the research findings are concluded. According to these findings, the ideal city from the perspective of the juridical teachings of Shiite is a city with a minimum diameter of one farsakh (about 5.5 kilometers) and the maximum diameter of four farsakhs (about 22 kilometers). In addition, the size of the city countryside can be up to four farsakhs.