The sizes of the field galaxies with I ≤ 25 have been measured in the New Technology Telescope (NTT) Deep Field. Intrinsic sizes have been obtained after deconvolution of the point-spread function with a multi-Gaussian method. The reliability of the method has been tested using both simulated data and Hubble Space Telescope (HST) observations of the same field. The distribution of the half-light radii is peaked at rhl 03, in good agreement with that derived from HST images at the same magnitude. An approximate morphological classification has been obtained using the asymmetry and concentration parameters. The intrinsic sizes of the galaxies are shown as a function of their redshifts and absolute magnitudes using photometric redshifts derived from the multicolor catalog. While the brighter galaxies with morphological parameters typical of the normal spirals show a flat distribution in the range rd = 1 - 6 kpc, the fainter population at 0.4 < z < 0.8 dominates at small sizes. To explore the significance of this behavior, an analytical rendition of the standard cold dark matter model for the disk size evolution has been computed. The model showing the best fit to the local luminosity function and the Tully-Fisher relation is able to reproduce at intermediate redshifts a size distribution in general agreement with the observations, although it tends to underestimate the number of galaxies fainter than MB ~ -19 with disk sizes rd ~ 1-2 kpc.