The resistive switching (RS) properties of strontium titanate (Sr1+xTi1+yO3+(x+2y), STO) based metal-oxide-metal structures prepared from industrial compatible processes have been investigated focusing on the effects of composition, microstructure, and device size. Metastable perovskite STO films were prepared on Pt-coated Si substrates utilizing plasma-assisted atomic layer deposition (ALD) from cyclopentadienyl-based metal precursors and oxygen plasma at 350¿°C, and a subsequent annealing at 600¿°C in nitrogen. Films of 15¿nm and 12¿nm thickness with three different compositions [Sr]/([Sr]¿+¿[Ti]) of 0.57 (Sr-rich STO), 0.50 (stoichiometric STO), and 0.46 (Ti-rich STO) were integrated into Pt/STO/TiN crossbar structures with sizes ranging from 100¿µm2 to 0.01¿µm2. Nano-structural characterizations revealed a clear effect of the composition of the as-deposited STO films on their crystallization behavior and thus on the final microstructures. Local current maps obtained by local-conductivity atomic force microscopy were in good agreement with local changes of the films' microstructures. Correspondingly, also the initial leakage currents of the Pt/STO/TiN devices were affected by the STO compositions and by the films' microstructures. An electroforming process set the Pt/STO/TiN devices into the ON-state, while the forming voltage decreased with increasing initial leakage current. After a RESET process under opposite voltage has been performed, the Pt/STO/TiN devices showed a stable bipolar RS behavior with non-linear current-voltage characteristics for the high (HRS) and the low (LRS) resistance states. The obtained switching polarity and nearly area independent LRS values agree with a filamentary character of the RS behavior according to the valence change mechanism. The devices of 0.01¿µm2 size with a 12¿nm polycrystalline stoichiometric STO film were switched at a current compliance of 50¿µA with voltages of about ±1.0¿V between resistance states of about 40¿kO (LRS) and 1 MO (HRS). After identification of the influences of the films' microstructures, i.e., grain boundaries and small cracks, the remaining RS properties could be ascribed to the effect of the [Sr]/([Sr]¿+¿[Ti]) composition of the ALD STO thin films.