In addition to the powers hereinbefore conferred upon all county boards, the board of supervisors shall have power (1) to change the boundaries of towns and to create new towns whenever the board determines that the existing towns are not workable towns and (2) to divide the county into convenient voting precincts and, as occasion may require, erect new ones, subdivide precincts already established, and alter voting precinct lines. When a voting precinct has less than seventy-five registered electors, the board of supervisors shall annex such voting precinct to another voting precinct except when the county is divided into more than two legislative districts. Any precinct having two hundred or more square miles and having more than twenty-five electors shall be excluded from being annexed to another voting precinct.