When it comes to aerodynamics the most drag is created by the rider(s).
A pilot's height has nothing to do with bar width; the broader the shoulders, the wider the bars.
As for the stoker, she/he doesn't have much choice. The width of stoker dropbars is determined by the width of the captains upper leg/butt . . . needs to fit between the bars.
Both bars on our Zona tandem are 42 cm; we are small/short 5'7" x 4'10 3/4".
On Rudy's racing single, bar width is 40 cm.
If you are really worried about aero . . . get tri/aero-bars.