High optical quality, well end leg faceted ZnO microtetrapods with leg length between 1 and 12 μm have been grown by carbothermal chemical vapor deposition. Lasing with mode quality factors of 2500-3000 is demonstrated. The origin of laser resonator cavity is discussed as a function of the tetrapod size. It is shown that in big tetrapods with legs of 12 μm in length the laser emission lines are well explained by longitudinal Fabry-Pérot modes generated in cavities formed by individual tetrapod legs. The dispersion of the ZnO refractive index is experimentally determined from the position of lasing modes in the temperature interval from 10 to 300 K. It is shown that the lasing mode structure is seriously affected by the decrease of the tetrapod size. For a small tetrapod with a leg length of 1 μm, the lasing modes cannot be explained anymore by the formation of longitudinal Fabry-Pérot modes in separate tetrapod legs, and the generation of guided modes by multiple total internal reflections in single tetrapod legs or in pairs of legs should be taken into account. The correlations between the lasing threshold and the tetrapod size are discussed.