Spatial spectra and characteristic scales of stratospheric density fluctuations obtained from the space station Mir observations of stellar scintillations are analyzed in this paper. The remote sensing method described is based on a well-known stellar scintillation phenomenon that arises when observing the stars through the Earth's atmosphere. To interpret scintillation spectra, a model of the three-dimensional (3-D) spectrum of atmospheric density fluctuations consisting of both turbulent and internal wave-associated spectral components is presented here. With the model chosen, we explain scintillation spectra at low frequencies by suggesting the atmospheric density fluctuations to be caused by a random ensemble of internal waves with the -5 power law decay for their 3-D energy spectrum. For the wave-associated anisotropic part of the spectrum, so-called outer and internal vertical scales are introduced to explain behavior of the observed scintillation spectra in their low-frequency range. These scales being earlier proposed only theoretically have been simultaneously revealed in the scintillation spectra presented here. The estimates of the outer scale for the different orbits of the space station are obtained and compared with those found from lidar and rocket soundings of the stratosphere. A possible cause of the observed variation in the wave number bandwidth of the wave-associated part of the spectrum of air density fluctuations is discussed.