An experimental approach for estimating the detonation cell size for liquid hypergolic propellants is presented and applied for monomethylhydrazine (MMH) and a MON-3 variant of nitrogen tetroxide (NTO) in a non-premixed combustor. The method utilizes a correlation between cell size and reactant fill height in an annular combustor geometry. Reactant fill height is inferred using a control-volume analysis to relate the geometry of the reactant fill zone to the propellant flow rate, detonation wave-speeds and number, and annular gap size. High-speed videography is used to measure the number of waves and their speed over a range of quasi-steady continuous detonation conditions. The detonation criteria is also proven valid in the transient conditions as the decrease of the fill height below a critical value identified in this work matches modal transitions with decreasing number of waves. A power law relating the cell width with induction length further supports the validity of the present technique down to cell sizes of 1–10 µm, a scale not practically resolvable with conventional methods.