Hi Don ;
In all actuality most folks look at this in different ways . Me , Now I look at it this way . If I want a really interesting model plane . I will so very lightly mist a darker color for panel lines . This said , you can't hardly see them Unless the light hits it right . Just like the real thing .
What I won't do now is specifically use a pen or paintbrush to show every little line in a color that pops , therefore making the plane look like a street map . In my natural metal birds I cut every panel from foil , just slightly larger than it's neighbor .
That way when I trim the overage away you have a distinctly visible , But , almost invisible line between . It's maybe less than the thickness of a sheet of paper . As far as actual size , well that has to be a judgement on the builders call .
If you put a lot of work into a model ship for instance , Say you have lines running everywhere representing welds .You better look at the real subject again . You cannot see lines of welds if you are ten feet away if the lighting is poor , or average , much less a hundred !
You see where I am going here ? Have a good one . Tanker - Builder