Like the Excelsior, the Mjolnir was designed to be both a test bed for experimental transwarp propulsion and a general purpose starship as well.
At 1.2 kilometer long, the Mjolnir is one of the biggest starships Starfleet has built. Its saucer section is actually a Photon Class scout ship connected by a large neck to the massive secondary hull which also holds the massive warp coils.
In the aft section is a large hangar capable of holding a Defiant size vessel, or several Raven/Vulture/Eagle class ships for individual exploration missions.