We report the evolution of the magnetic penetration depth (lambda) and superconducting energy gap (Delta) in epitaxial NbN films with thickness (d) varying between 51-3 nm. With decrease in film thickness T-c and Delta(0) monotonically decreases, whereas lambda(0) monotonically increases. Our results show that while the values of Delta(0) and lambda(0) are well described by Bardeen-Cooper-Schrieffer theory, at elevated temperatures, films with d < 6.5 nm show sudden drop in superfluid density associated with the Kosterlitz-Thouless-Berezinski transition. We discuss the implication of these results on the time response of superconducting bolometers made out of ultrathin NbN films.