http://web.stanford.edu/class/history5n/FPmemo.pdf

The critical radius r0-i.e., the radius of a sphere in which the surplus of
neutrons created by the fission is just equal to the loss of neutrons by escape
through the surface-is, for a material with a given composition, in a fixed
ratio to the mean free path of the neutrons, and this in turn is inversely proportional
to the density. It therefore pays to bring the material into the densest
possible form, i.e., the metallic state, probably sintered or harnmered. If
we assume, for uranium 235, no appreciable scattering, and 2.3 neutrons
emitted per fission, then the critical radius is found to be 0.8 times the mean
free path. In the metallic state (density 15), and assuming a fission crosssection
of 10 -23 cm2,’ the mean free path would be 2.6 cm and r0 would be 2.1
cm, corresponding to a mass of 600 grams. A sphere of metallic uranium 235
of a radius greater than r0 would be explosive, and one might think of about 1
kg as a suitable size for the bomb.