Excuse me if this feature(s) have been suggested.
Currently, Slic3r 0.7.1 sets the extrusion width ratio automatically based on the layer height setting. I've found many times when I don't really care about the layer height but I would like to specify the extrusion width. One example is when I want to print a part where I want a thin wall to be exactly two filament layers. If the thin wall is 1 mm wide, setting the extrusion width to 0.5 mm should make the walls two filaments wide. It would be easier in such cases to set the layer height to 0 (auto) and the extrusion width to to 0.5 mm. This is similar to how cameras allow manually setting the shutter speed or the f-stop in semi-automatic mode.
Another feature that would be worth considering would be a fully automatic mode where both layer thickness and extrusion width are set to 0 and slic3r automatically selects them based on nozzle diameter and printing speed.
In order to do this, the extrusion width setting need to be changed from ratio to mm.
Thanks.