Width over height
Width over height is the ratio of the layer height to the width of the lines laid down. Experimentally, it seems as if a ratio of < 2 produces brittle parts. A ratio > 3 is going to give up detail (owing to the line width being thicker).
In Slic3r, you can set this by setting the extrusion width to be some %, which is calculated relative to the layer height. So if you want a W/H of 2.2, set your extrusion width to 220%.