Contracts for the laying of such sidewalks and roadways as are provided for in section 231.400 shall be authorized and let by the aforesaid county commission to the lowest and best bidder therefor, said commission first giving notice thereof by publication, which notice shall be published for fifteen days in a daily newspaper or for three insertions in a weekly newspaper published at the county seat of such county. Said notice must contain specifications showing the kind of material required, the width and the thickness of such sidewalk and roadway and the manner of paying therefor; provided, that all sidewalks constructed under the provisions of sections 231.400 to 231.430 shall be of uniform width not less than four feet in any one block, and the general specifications of such sidewalk or sidewalks shall be determined by the county commission in conformity with the wishes of the petitioners.