here in Sketchup gives the biggest segment from a selection of segments

This one-liner copy+paste + <enter> in the Ruby Console highlights the longest segment in the selection and prints its length/id

By TIG

d=0;s=Sketchup.active_model.selection;a=s.to_a;q=nil;a.each{|e|(q=e;d=e.length) if e.class==Sketchup::Edge and e.length>d};s.clear;s.add(q);puts d;q

About the problem : generally perspective has a cone or a rectangle of visibility
Here your curves are coplanar at the place where you want find the biggest segment ?