We report on the synthesis and optical gain properties of regularly shaped lead iodide (PbI2) platelets with thickness ranging from 10 to 500 nm using chemical vapor deposition (CVD). The as-prepared single crystalline platelets exhibit a near band edge emission of ~500 nm. Whispering gallery mode (WGM) lasing from individual hexagonal shaped PbI2 platelets is demonstrated in the temperature range from 77 to 210 K - where the lasing modes are supported by platelets as thin as 45 nm. The FDTD simulation and the edge-length dependent threshold confirm the planar WGM lasing mechanism in such hexagonal shaped PbI2 platelet. Through a comprehensive study of power-dependent photoluminescence (PL) and time-resolved PL spectroscopy, we ascribe the WGM lasing to be biexcitonic in nature. Moreover, for different thickness of platelet, the lowest lasing threshold occurs in platelets of ~120 nm, attributing to the formation of a good Fabry-Pérot resonance cavity in the vertical direction between the top and bottom platelet surfaces that enhance the reflection. Our present study demonstrates the feasibility of planar light sources based on layered semiconductor materials and their thickness dependent threshold characteristic is beneficial for the optimization of layered material based optoelectronic devices.