We present a study of the H II regions in the central area of the giant irregular galaxy NGC 4449 based on observations taken on the William Herschel Telescope at the Roque de los Muchachos observatory with the Taurus II Fabry-Perot interferometer in the emission lines of Hα and [O III] 5006.8. We show that the usual methods of determining sizes of H II regions are not valid in this galaxy because of the high surface density of nebulae and the presence of an intense diffuse luminosity over NGC 4449, so it has been necessary to develop a new method. The procedure we have used allows integrated parameters of H II regions (radius, luminosity, radial velocity, and velocity dispersion) to be obtained with greater objectivity and precision than with other methods. The technique proposed here can be extended to parameterize giant H II regions in galaxies in general and to resolve the problem of the large uncertainties associated with previous methods in characterizing star-forming regions in Magellanic irregular galaxies.