Eclipse was 17,500 meters
Executor was 19,000 meters
The New Republic did capture some though, I believe they captured 3 (not sure if there were more), the Guardian, Eclipse II and the Lusiankya I think.
They also produced some 8 KM SSDs, though those may as well have been Super Class Star Destroyers this was during the Yuuzong Vong war.
Oh and the Republic Class Star Destroyer, which may as well have been in the MC series.