40KT to 45MT, upper end much cleaner!
The value of a prolate primary lies apparently in the fact that a MIRV warhead is limited by the diameter of the primary — if a prolate primary can be made to work properly, then the MIRV warhead can be made considerably smaller yet still deliver a high-yield explosion — a W88 warhead manages to yield up 475 kt with a physics package 68.9 in (1.75 m) long, with a maximum diameter of 21.8 in (0.55 m), and weighing probably less than 800 lb (360 kg).
Other references indicate a low end of 350 KT and high end (theoretical) of about 4.8 MT.
Heckava flash bulb.