I would have to guess that the stars in the constellation Delphinus win this contest. The entire constellation is only about 3 degrees long. The stars that make up this constellation are:
Star.........Type........Distance................Velocity alpha B9 IV -6 km/sec beta F5 IV 125 light years -23 gamma F7 V 100 -8 delta A7 III +9 epsilon B6 III -19 zeta A3 V -22 eta A3 IV -18 theta K3 Ib -14 iota A2 V -4 kappa G5 IV -52What information I could find is sketchy, but it seems like there may be two collections of stars in this constellation. One subgroup has a velocity near 0 km/sec while the second has a velocity near -20 km/sec. The average visual magnitudes for the V and IV-class stars is about +5.0, and with absolute magnitudes for these stars near +1.5 this makes their distances roughly M - m = -5 + 5logd(pc) or 50 parsecs, or 160 light years. Theta Delphi, however, is a supergiant and is much farther away than the other stars making it definitely not a physical member of this constellation. At a distance of 160 light years, the 3-degree extent of this cluster on the sky equals about 2.8 light years. Still, the differences in the distances to the individual stars may be much greater than this. Still, it does LOOK like a very compact asterism.