The Theta-class raider was equipped with deflector shields and armed with a dual forward photon torpedo bank and two wing mounted cannons for launching concentrated plasma charges. With this weaponry it could easily outgun small ships, such as Starfleet Danube-class runabouts. Its maximum velocity was capable of exceeding warp 5. (DS9: "Vortex")
Ships of the class
This studio model was designed by Ricardo F. Delgado, and was described on his concept art as a "raptor". Notes on the camera test model of Baran's ship indicate this ship to be "1/4 length of Enterprise", which suggest an approximately length of 160 meters. 
In "Vortex", the runabout's computer identified Ah-Kel's raider as "Miradorn vessel, Theta-class", suggesting that the ships of this class are constructed by the Miradorn.
Baran's mercenary vessel, later seen in TNG: "Gambit, Part I" and "Gambit, Part II", was not specified as a Theta-class ship in those episodes, but based on its numerous similarities, unmodified studio model and use as a mercenary ship, and moderate top speeds, suggest it is also a ship of this class.
The CCG: Second Edition named this type of vessel as a Ma-Karn-class ship.