Sometimes rock stimulations by fluid injections into geothermal boreholes are able to trigger perceptible or even potentially damaging earthquakes. This does not seem to be the case for hydraulic fracturing of hydrocarbon reservoirs. Reasons for such a difference and factors defining magnitudes of induced earthquakes (triggered tectonicly as well as induced artificially) remain unclear. We analyzed microseismic data obtained by fluid stimulations at different geothermal and hydrocarbon sites. This analysis indicates that a rupture corresponding to a fluid-induced earthquake seems to be only probable along a surface located mainly inside a stimulated rock volume. We approximated the stimulated volume by an ellipsoid, and compared the statistics of induced events with the statistics of randomly distributed thin flat disks modeling rupture surfaces. We found that one of the main factors limiting the probability to induce a large-magnitude event is the minimum principal axis of a fluid-stimulated rock volume. This geometrical scale can be very different in geothermal and hydrocarbon reservoirs. It may control the order of a largest possible magnitude. We quantified an impact of the geometry of a stimulated volume on the Gutenberg-Richter-type frequency-magnitude distribution of induced earthquakes. Our results show that monitoring the spatial growth of seismicity in real time can help to constrain a risk of inducing damaging earthquakes.