This paper addresses the prediction and measurement of the aggregation number and radius of spherical micelles composed of telechelic polymers. The free energy of micellization balances the interfacial energy against the energy of stretching the end blocks to form the core, the configurational entropy, and excluded-volume interactions of the soluble chains that form the corona. In a good solvent, the latter are strongly stretched and form spherical brushes as treated by the Li-Witten model. The minimum in the free energy sets the most probable size, while the curvature at the minimum indicates the breadth of the distribution. From these we predict the various averages of the aggregation number and micellar radius detected by fluorescence measurements, dynamic and static light scattering, and viscometry. The effects of polydispersity explain in part the discrepancies among values reported by different groups for polymers of the same nominal structure.