The structure of poly(methyl methacrylate)-block-poly(n-butyl methacrylate) (PMMA-b-PnBMA) micelles in the room temperature ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]), a selective solvent for the PMMA block, has been studied using dynamic light scattering (DLS) and small-angle X-ray scattering (SAXS). A series of seven PMMA-b-PnBMA diblock copolymers, were prepared by reversible addition-fragmentation chain-transfer (RAFT) polymerization, in which the degree of polymerization of the PMMA block was kept constant while the PnBMA block length was varied. All the polymers formed spherical micelles at ambient temperature in dilute solution; their hydrodynamic radius (R-h) and core radius (R-c) were obtained by DLS and SAXS, respectively. It was found that R-c and the degree of polymerization of the core block, N-B, followed a power law relationship in which R-c similar to N-B(0.71 +/- 0.01). The corona thickness (L-corona), given by the difference of R-h and R-c, does not show any apparent dependence on N-B. These results were compared to scaling theory, and were found to be only in partial agreement with the star model proposed by Halperin et al. However, the mean-field calculations of micellar dimensions by Nagarajan and Ganesh were in excellent agreement with the, data. This comprehensive experimental study, provides precise quantification of the R-c and L-corona dependence on core block:lengths, due to the use of seven different block copolymers with identical corona block lengths.