The invention relates to dimensioning rules for a blade of an axial flow impeller. The dimensioning rules relate to: (i) the lengthwise dimension from the axis of rotation of the impeller to the tip of the blade; (ii) the widthwise dimension of the blade perpendicularly to the lengthwise direction; and (iii) respective angles for a plurality of folds of the blade. The invention also relates to an axial flow impeller having such blades.