A regular grand jury shall consist of not less than five nor more than seven persons. Each grand juror shall be a citizen of this Commonwealth, eighteen years of age or over, and shall have been a resident of this Commonwealth one year and of the county or corporation in which the court is to be held six months, and in other respects a qualified juror, and, when the grand juror is for a circuit court of a county, not an inhabitant of a city, except in those cases in which the circuit court of the county has jurisdiction in the city.