Based on the second-order moment of the power density,the explicit expressions for the beam width and far-field divergence angle of nonparaxial flattened Gaussian(FG) beams are derived. It is found that as w₀/λ(waist width-to-wavelength ratio)→0,the far-field divergence angle approaches an asymptotic value of θ_max=63.435°,indenpent of the beam order. Then,the nonparaxial Gaussian beam instead of the paraxial one is chosen as the ideal beam,the M² factor of nonparaxial FG beams is studied and compared with the conventional definition of the M² factor. In the nonparaxial regime,the M² factor of nonparaxial FG beams depends not only on the beam order,but also on w₀/λ. According to our definition,as w₀/λ→0,the M² factor of nonparaxial FG beams does not equal to zero,but approaches 0.913,0.882 and 0.886 for the beam orders N=1,2,3,respectively. In particular,for N→∞ the M² factor takes its minimum M²_min=0.816.