The lipid raft hypothesis describes ordered domains of cell membranes, enriched in cholesterol, sphingolipids, and saturated phospholipids, that are responsible for a number of cellular processes such as lipid sorting, protein signaling, and signal transduction. These lipid rafts are highly mobile, and it is believed that their coalescence and separation is what drives these cellular processes. For this reason, cellular function depends not only on the presence of lipid rafts but also on raft size. This work uses steady-state fluorescent techniques, specifically Förster Resonance Energy Transfer (FRET) along with a mathematical model to detect and estimate the size of lipid domains in membrane systems modeling natural cell membranes. The fluorescence study was initially performed on a simple two-component phospholipid and cholesterol system in order to characterize the behavior of the fluorescent probes using an established phase diagram and was then extended to more complex, three-component systems. In these ternary systems, the effect of phospholipid type, sterol type, and sterol composition on domain size was determined. It was found that very slight changes in the composition of the membrane (i.e., both type and amount of “line-actant”) have significant effects on domain formation and size by changing the amount of line tension present in the membrane.