Integration of optically active materials into whispering gallery mode (WGM) cavities enables low-threshold laser emission. In contrast to their passive counterparts, the WGMs of these microlasers can be pumped and read out easily via free-space optics. The WGMs interact with the cavity environment via their evanescent field, and thus lend themselves to label-free bio-sensing. The detection limit of such sensors, given as the ratio of the resolution of the whole measurement system to the sensitivity of the WGMs, is an important figure of merit. In this work we show that the detection limit of polymeric microdisk lasers can be improved by more than a factor of seven by optimizing their radius and thickness. We use the bulk refractive index sensitivity, the magnitude of the sensor reaction towards refractive index changes of the bulk environment, to quantify the sensing performance and show that it can be enhanced while the spectral resolution is maintained. Furthermore, we investigate the effect of the size of the cavity on the quality factor and the lasing threshold in an aqueous environment, hence allowing optimization of the cavit ... mehry size for enhanced sensor performance. For all considered quantities, numerically computed expectations are verified by experimental results.