Our previous model calculations [cf. : Bergstrom, M.; Eriksson, J. C. Langmuir 1996, 12, 624] together with a multiple chemical equilibrium approach indicate that the spontaneous formation of geometrically closed bilayer vesicles is geared primarily by the bilayer tension which in turn is largely determined by the work of bending the bilayer into a spherical vesicle and a statistical-mechanical factor that accounts for the fluctuations in composition, chain packing density, and shape. On this basis, we obtain vesicle size distributions with maxima located at radii several orders of magnitude larger than where the local free energy minimum of a single equilibrated vesicle actually occurs. Moreover, according to our analysis, the relative width of a vesicle size distribution, sigma(R)/R-max, is generally equal to 0.283 for equilibrated vesicle dispersions, independently of the energetic vesicle parameters. An explicit relation between the bilayer bending constant and the most probable vesicle radius R-max is also derived, showing a dramatic increase in size with the work of bending a planar bilayer into a vesicle. Employing our previous calculations of the bending work of mixed sodium dodecyl sulfate/dodecanol bilayers, we predict vesicles with R-max in the range 0.1-10 mu m, in qualitative agreement withrange 0.1-10 mu m, in qualitative agreement with experiments.