Register UK - DARPA would like to see portable nuclear reactor proposals able to carry a 5 to 10 megawatt electrical load in addition to producing 15,000 gallons of JP-8 or road fuel daily - enough to fill up a Chinook helicopter around a dozen times.
They also want designs which are "inherently safe (negative temperature coefficient)", ie ones which can't suffer a runaway if damaged and melt their way down out of the base into a glowing tunnel in the Earth's crust.
As for size, DARPA doesn't offer any detailed requirements, but it does say that the reactor and its associated generators, fuel plant etc should be "readily deployable". That probably won't be too hard; existing submarine reactors in the same power range are only dustbin-sized.
The Pentagon tech-heads also aim to deal with proliferation fears by specifying that neither the reactor's fuel nor its wastes should be of any use in making weapons: thus, even if enemy forces managed to overrun a US base or steal its power'n'jetfuel plant they'd be no further foward toward making a bomb (or anyway no further than any nation with a civil nuclear programme.
It seems that DARPA is asking for the Hyperion Power Generation factory mass produced reactor which might be ready in 2013.
There are several other small modular nuclear reactors that in development from Nuscale and Toshiba.
Dark Horse Possibility
If Lawrenceville Plasma Physics can convince DARPA to take a longer shot then the dense plasma focus nuclear fusion reactor could be a match. LPP is looking to make 5 MW nuclear fusion reactors.