We report a detailed photoluminescence (PL) study on single silicon nanocrystals produced by laser pyrolysis. The PL spectra reveal nearly homogeneously broadened zero-phonon lines coupled to Si-O-Si phonon transitions in the SiO2 shell. A systematic investigation of electron-phonon coupling is reported on the basis of single nanocrystals. The stepwise localization of electron and hole at the Si-SiO2 interface for nanocrystals smaller than d approximately 2.7 nm is driven by electron-phonon coupling. From the localization energies the effective Bohr radii of the (localized) electron and hole are estimated to be in the range of 1-2 bond lengths of Si-O and Si-Si.