Quantitative ferromagnetic resonance (FMR) measurements at 9.5 and 35 GHz were made on lunar fines in order to determine the amount of single domain Fe0 and to ascertain the effect of sample quantity on the results of these experiments. Samples containing more than 10μg of Fe0 in an X-band TE104 cavity, or more than 0.11 μg in a K0-band TE011 cavity can cause errors greater than 1% in the quantitative determination of Fe0, as well as cause anomalous broadening of the FMR line. Theoretical calculations were derived to show this effect which results from a nonlinear response of the FMR experiment with the sample quantity. Using samples o nominal weights, the amount of Fe0 determined at 35 GHz was 1.2--1.8 times greater than at 9.5 GHz, while the linewidths increased by a factor of 1.2. As the particle size increases, the ratio of FE0 determined at 35 GHz to that determined at 9.5 GHz increases, suggesting that the additional iron is due to larger spheroidal particles which change from multidomain to single domain at the larter fields.