The Wide Field Imager (WFI), one of two complementary instruments on board ESA’s next large X-ray mission Athena, combines state-of-the-art resolution spectroscopy with a large field of view and high count rate capability. Centerpiece of the WFI instrument are four DEPFET (Depleted p-channel FET) detectors with a size of 512×512 pixels each. Their size involves challenging demands on the readout process, concerning timing and homogeneity over such large scales. In order to estimate the influence of flight size, smaller prototype sensors of various size, chosen to address the specific problems, were produced and characterized. Both possible readout modes, source follower and drain current readout, were investigated regarding their specific size dependent problems. Time resolution of the particularly stable source follower readout is limited by RC time constants and the fast drain current mode is restricted by inhomogeneities of the pixel matrix, demanding specific properties of the Veritas readout ASIC. To characterize these effects, spectroscopic measurements, showing good performance of the devices, were complemented with electrical ones, enabled by features of the Veritas-ASIC. We characterized the time dependency of the source potential and its influence on signal offset in source follower mode as well as the influence of drain current distribution on detector properties in drain current mode. The limits and capabilities of the current to voltage converter stage were also investigated. Performance measurements on large prototype detectors using photon energies of 5.9 keV show promising results.