  We define a physically-motivated measure for galactic bar length, called the
dynamical length. The dynamical length of the bar corresponds to the radial
extent of the orbits that are the backbone supporting the bar feature. We
propose a direct observational technique using integral field unit spectroscopy
to measure it. Identifying these orbits and using the dynamical length is a
more faithful tracer of the secular evolution and influence of the bar. We
demonstrate the success of the metric for recovering the maximal bar-parenting
orbit in a range of simulations, and to show its promise we perform its
measurement on a real galaxy. We also study the difference between
traditionally used ellipse fit approaches to determine bar length and the
dynamical length proposed here in a wide range of bar-forming N-body
simulations of a stellar disc and dark matter halo. We find that ellipse
fitting may severely overestimate measurements of the bar length by a factor of
1.5-2.5 relative to the extent of the orbits that are trapped and actually
comprise the bar. This bias leads to overestimates of both bar mass and the
ratio of corotation radius to bar length, i.e. the bar speed, affecting
inferences about the evolution of bars in the real universe.
