Flowers of Verbascum nigrum L. (mullein) contain considering amounts of saponins which are surfactants with carbohydrates chain in their molecules. Hence, the mullein was used for water-oil (W/O) systems as an emulsion stabilizer. As an oil phase paraffin, rapeseed and Vaseline oils were used respectively. Emulsification and foaming tests were performed to evaluate stability of the emulsion system. Emulsification indexes (EI) were obtained after 24 h of W/O phases mixing. Moreover, EI indexes were determined after 8 weeks (E8w) of emulsions storage at room temperature. The impact of the plant extract particles size was evaluated too. Dynamic light scattering (DLS) and nanoparticle tracking analysis (NTA) techniques were used for the hydrodynamic diameter (dH) measurements (average values of dH were 133 nm and 92 nm for DLS and NTA analysis respectively). Raman spectra of mullein extracts and zeta potential (ζ = −43.3 mV) were measured and compared with another selected saponins plant extract. The emulsification tests showed that the emulsion stability increased with the decreasing size of the surfactant (mullein) hydrodynamic diameter. The best stability of the investigated O/W emulsion was shown for Vaseline oil system. Moreover, high impact on the surfactant properties of V. nigrum L. has the particle size. The O/W emulsions stabilized by filtrated extract nanoparticles exhibit longer stability than crude extract. The surface properties and nanostructures size proved that mullein extracts can be considered as a potential stabilizer for hydrocolloid systems and dedicated for food industry.