Fluorescence resonance energy transfer (FRET) is sensitive to lateral heterogeneity in multicomponent membranes. Recently, we developed a model that enables the extraction of domain size from time-resolved FRET data, and here we examine the effects of domain polydispersity and spatial ordering on the accuracy of domain size determination. The model is applied to Monte Carlo calculations of membranes containing polydisperse domains, either randomly or hexagonally packed for three probe-partitioning schemes and three domain surface coverages. Statistical analysis of the model fitting results supports the conclusion that time-resolved FRET is a robust and precise technique for determining the size of small membrane domains (<= 4 times the Forster distance, similar to 20 nm, of the FRET pair), regardless of domain polydispersity and packing. The confidence estimates, presented here in dimensionless domain diameters (independent of the specific FRET pairs), provide a universal tool for domain size estimation in systems containing membrane-bound fluorophores.