Barium and Strontium Ferrites exhibit strong magnetic anisotropy at millimeter waves due to their hexagonal configuration. Barium and Strontium Ferrite powders with different particle diameter are measured in millimeter wave frequency range. A Quasi-optical spectrometer in transmission mode powered by high power backward oscillator is employed to measure the transmittance of these powder samples. The ferromagnetic resonance is determined from the transmittance spectra. For the nano-size powders the dip in the transmittance becomes weaker and sharper. The dip or the minimum in transmittance spectrum which is essentially the resonance peak moves to lower frequency. It is mainly due to the decrease in domain size. Possible errors in the evaluation of permittivity and permeability are analyzed.