A traditional road bar isn't flared and has a straight section at the ends of the drops, which is set parallel to the ground to define a default bar rotation for the purpose of measurement and new bike setup.
From that rotational angle, the conventional way of measuring it is:
Width = center to center at the ends of the bars
Reach = horizontal dimension of center of bar at stem clamp to center of bar at forward-most point
Drop = vertical dimension of center of bar at stem clamp to center of bar at end of drop
A lot of current bars don't have the clearly delineated straight section at the ends of the drops that is used to communicate a default angular position as above to measure from. Of these, it's pretty common but not universal for the orientation of the graphics to be an attempt at doing the same job, i.e. when they're pointing straight ahead, that's your default angle. Other times you have to play games with angling the bars such that the measurement of the drop and the reach suddenly both become a whole number in CM, and that's the angle to measure from. Other times you have to just arbitrarily set it to what looks the most reasonable. It's an inexact thing on many of the swoopier contemporary bars, which is annoying.
For measuring width on a flared bar, you're just holding a tape measure across the tops at the beginning of the bends. This is almost always going to be near a whole number in CM, which is what the bar is "trying to be."