We report on the combined use of fluorescence correlation spectroscopy (FCS) and H-1 and C-13 NMR spectroscopy to detect the size and type of peptide secondary structures in a series of poly-Z-L-lysine functionalized polyphenylene dendrimers bearing the fluorescent perylenediimide core in solution. In dilute solution, the size of the molecule as detected from FCS and H-1 NMR diffusion measurements matches nicely. We show that FCS is a sensitive probe of the core size as well as of the change in the peptide secondary structure. However, FCS is less sensitive to functionality. A change in the peptide secondary conformation from beta-sheets to alpha-helices detected by C-13 NMR spectroscopy gives rise to a steep increase in the hydrodynamic radii for number of residues n >= 16. Nevertheless, helices are objects of low persistence.