Take a look at my answer to a related question Proof of part of haversine formula?
The first part is directly relevant:
The radius, r, of the small circle joining all points at latitude, φ is
r = R cos φ
where R is the radius of the sphere.
However, instead of the chord length of a straight line joining two points on the same latitude (a requirement of that question), you want the simpler arc length of part of the small circle. It is simply
AD = r dλ
where dλ is the difference in longitude of A and D.
As always in spherical trigonometry, things get more complicated in the case of an ellipsoid. The first equation above relates only to the globe being a simple sphere.
Assuming an Earth radius of 6,371km, the above formulas yield a result of 15,758km for the rhumb-line distance between your two points.
