The only canonical bit of info on this class is that it was in UFP Starfleet service in the 2160s and remained so until 2196, and that USS Essex and her crew of 229 were part of this service (TNG "Power Play").
Backstage sources have tried to suggest that the class looks like the tabletop model sometimes seen in DS9, with a sphere for a primary hull and a cylinder for the secondary. This is pure speculation, though. And the design of that tabletop decoration looks a tad too small for a ship supposed to hold 229 people.
It would be fun to speculate that the class indeed came to be in the 2140s, because an ENT novel by Dave Stern, Daedalus, speaks of a prototype ship of that name that is (supposed) lost in 2141 or so. Other ENT novels then speak of assorted Daedalus class ships as previous assignments for the 2150s heroes, mainly the MACOs. So perhaps the design wasn't a complete failure despite the loss of the prototype, and continued service right through the transformation of the UE Starfleet to the UFP Starfleet, gaining NCC registries in the process.
FWIW, Dave Stern's Daedalus tried to test a radical new power production mechanism he called the Cascade Ion Drive. Apparently this didn't pan out, because our ENT heroes imply it is not in Starfleet use (although the Suliban seem to be using it), but that doesn't mean that the whole ship class fizzled. The series production Daedaloi may have been propelled by a more conventional powerplant.
Timo Saloniemi