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 almost a kilometer long, the Mjolnir is one of the biggest starships Starfleet has built. It’s saucer section is integrated into 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.