An explicit "step range" makes no sense for Linear Scaling, because values w are calculated using the formula
w = ((v - smin)/(smax - smin)) * (tmax - tmin) + tmin
with v the property value of the graph element and smin,smax,tmin,tmax the four values you have to specify in the mapping's conversion details.
Moreover, yEd only supports very few line widths. E.g. for solid lines the widths 1, 2, 3, 4, 5, 6, 7 are supported, for the other line types only widths 1, 2, 3, 4, 5 are supported. Therefore varying line widths are simply not well suited to visualize a large range of property values.