23 In revising diocesan boundaries first place must be accorded to organic unity of each diocese, with due regard to the personnel, the offices and institutions, which form, as it were, a living body. In individual cases all circumstances should be carefully studied and the general criteria which follow should be kept in mind. 1.) In determining a diocesan boundary, as far as possible consideration should be given the variety in composition of the people of God, for this can contribute greatly to a more effective exercise of the pastoral office. At the same time the natural population units of people, together with the civil jurisdictions and social institutions that compose their organic structure, should be preserved as far as possible as units. For this reason, obviously, the territory of each diocese should be continuous. Attention should also be given, if necessary, to civil boundaries and the special characteristics of regions and peoples, such as their psychological, economic, geographic and historical backgrounds. 2.) The extent of the diocese and the number of its inhabitants should generally be such that, on the one hand, the bishop himself- even though assisted by others-can officiate at pontifical functions, make pastoral visitations, faithfully direct and coordinate all the works of the apostolate in the diocese and know well especially his priests, and also the religious and lay people who are engaged in diocesan projects. On the other hand, an adequate and suitable area should be provided so that bishop and clergy, mindful also of the needs of the universal Church, can usefully devote all their energies to the ministry. 3.) Finally, in order that the ministry of salvation be more effectively carried out in each diocese, it should be considered a general rule that each diocese have clergy, in number and qualifications at least sufficient, for the proper care of the people of God; also, there should be no lack of the offices, institutions and organizations which are proper to the particular church and which experience has shown necessary for its efficient government and apostolate; finally, resources for the support of personnel and institutions should be at hand or at least prudently foreseen in prospect. For this same purpose, where there are faithful of a different rite, the diocesan bishop should provide for their spiritual needs either through priests or parishes of that rite or through an episcopal vicar endowed with the necessary faculties. Wherever it is fitting, the last named should also have episcopal rank. Otherwise the Ordinary himself may perform the office of an Ordinary of different rites. If for certain reasons, these prescriptions are not applicable in the judgment of the Apostolic See, then a proper hierarchy for the different rites is to be established. Also, where similar situations exist, provision should be made for the faithful of different language groups, either through priests or parishes of the same language, or through an episcopal vicar well versed in the language-and if needs be having the episcopal dignity- or at least in some other more appropriate way.
Source: Christus Dominus (Vatican.va)