Particle size and surface trap effects on the third order nonlinear optical (NLO) properties of CsPbCl3 and CsPbBr3 perovskite quantum dots (PQD) have been investigated using femtosecond (fs) Z-scan experiment. Firstly, optical, morphological and structural characterizations of the synthesized PQDs were performed using linear absorption and photoluminescence (PL) spectra, XRD and TEM methods. Next, NLO properties of CsPbCl3 and CsPbBr3 PQDs with three different particle sizes and with and without surface trap states were studied at the operating wavelength of 800 nm and 3.564 x 1011 W/cm2 intensity. The calculated values of the nonlinear absorption coefficient, β, (~10-11 m/W), the nonlinear refraction coefficient,γ, (~ 10-17 m2/W), and third order susceptibilities, χ(3), (~ 10-11 esu) for both sample types were seen to be in line with the literature. The maximum γ values for CsPbCl3 and CsPbBr3 PQDs were observed in the large and medium particle sizes, respectively. In the case of surface trap free nanomaterials, the unitless figure of merits were found to be increased more than 100 folds for CsPbCl3 PQDs and 30 folds for CsPbBr3 PQDs. These results strongly indicate the significant potential of CsPbCl3 and CsPbBr3 PQDs for numerous nonlinear applications.