The size of a spiral core is predicted on the basis of a boundary-layer analysis of a spiral tip in the nonkinematic limit. Dependencies of a spiral core radius on medium parameters are found analytically and numerically for two models of excitable media. It is shown that the core radius is determined by the conditions for the tip separation from a core boundary. It is found that a core radius depends on all currents that are active during a wave front formation associated with the diffusion fluxes coupling neighboring elements of excitable media. Varying currents that activated only after wave front formation do not result in a significant altering of a spiral core radius. @S1063-651X~97!51010-5# PACS number~s!: 47.32.Cc, 42.15.Dp, 82.40.2g