Poly(butadiene-ethylene oxide) (PB-PEO) block copolymers form wormlike micelles in water. These structures can be characterized over a very broad range of length. scales, ranging from the contour and persistence length to the core/corona diameter and the total aggregation number. Here we use fluorescence microscopy, dynamic and static light scattering and small angle neutron scattering to obtain information on the full range of length scales, while changing the solvent selectivity of the block copolymer by varying the molar ratio of mixtures of N,N-dimethylformamide (DMF) and water. We show that the wormlike micelles become very small and flexible before a transition to spherical micelles sets in decreasing the interfacial tension. We calculated the free energy contributions for the different species using the experimental input and confirmed that the transition to spherical micelles is energetically favored.