We have developed a model that describes the ferromagnetic resonance spectra recently observed in nanotubes formed by assembled La_0.67Sr_0.33MnO₃ manganite nanoparticles. The resonance line shape in these tubes resembles that of a system of randomly oriented flat particles instead of what is usually seen in elongated samples. In order to explain the experimental data we have assumed that each individual grain (or small group of grains) has an easy plane effective anisotropy which could be partially aligned in a cylindrical symmetry. The magnetization of each grain is also distributed around a mean value, and hence the magnitude of the anisotropy varies from grain to grain. Within this framework we have been able to fit reasonably well the resonance field and the overall line shape of nanotubes that were deposited on a glass substrate and were aligned in a magnetic field. From the simulations we have deduced an average effective magnetization M = 180 emu/cm³ with a distribution width of 140 emu/cm³. This value of magnetization is smaller than that of bulk LaSr manganites at room temperature, probably due to the granular nature of the tubes.