[aerohead]

I've added four additional pictorial drag tables up at the top of the thread.
Of particular interest to motorcyclists will be the 'wing/strut sections' table.
The center section has the lowest drag of traditional symmetrical sections,with a thickness-to-chord ratio of 3.92.
The upper,very thin section suffers from an overdose of surface friction drag.
The bottom 'stubby' section suffers from separation-induce pressure drag.
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The center lowest-drag section is at the crossroads of the drag curves for surface friction and pressure drag.It's right at the 'sweet spot.'
If the section is any shorter,it suffers separation.
If it's any longer,it's surface friction begins to climb.
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From a purely aerodynamic standpoint,your streamlined bike should be as near to the center profile as you can make it,when viewed from above.
The drag curve in this region is rather flat,like a nice torque curve with a stump-pulling engine.So you've got some wiggle room dimension wise.But you won't want to stray too far afield.