The goal of this study was to analyse the effect of sonoreactor dimensions on the effectiveness of the ultrasound-assisted extraction (UAE) of bioactive substances from rowan (Sorbus aucuparia L.). Sonication was carried out with a VC750 Sonics processor at the following amplitudes of ultrasound: 12, 24, and 36 µm. The frequency of the ultrasound was 20 kHz. Extraction was conducted in a 2 s on–4 s off pulse system. The total phenolic content and antioxidant activity were determined using a spectrophotometric method. The pH value of the extracts was measured using a combined pH metric electrode, type EPS-1 (Elmetron). Response surface methodology (RSM) was used to optimise the investigated variables. On the basis of the developed model, the following variable values were obtained: TPC—12.48 gallic acid equivalent (GAE)/g, 2,2′-azino-bis-(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) values—126.54 μmol Trolox (TE)/g, 2,2-diphenyl-1-picryl-hydrazyl-hydrate (DPPH) values—79.58 μmol TE/g, ferric reducing antioxidant power assay (FRAP) values—120.65 μmol TE/g for an extraction vessel with a diameter of 35 mm and TPC—11.68 mg GAE/g, ABTS values—120.49 μmol TE/g, DPPH values—75.90 μmol TE/g, FRAP values—155.54 μmol TE/g for an extraction vessel with a diameter of 25 mm. Significant influences of time, ultrasound amplitude, and alcohol concentration on the course of the extraction process in the tested extraction vessels were found. The optimum extraction conditions for an extraction vessel with a diameter of 35 mm were obtained for an ethyl alcohol concentration of about 60%, and for an extraction vessel with a diameter of 25 mm the optimum conditions were for an alcohol concentration in the range of 30–50%. A very strong positive linear correlation was found between the concentration of ethyl alcohol and the pH value of the obtained extracts for both extraction reactors. The developed models of pulsed ultrasound-assisted extraction were characterised by a good predictive capacity (77.49–91.39%) and can be used for obtaining polyphenols from rowan.