Three co-flow jet (CFJ) airfoils with injection slot size differed by two times consecutively are calculated by using a RANS CFD solver with 1-equation Spalart-Allmaras model. At the same angle of attack(AoA), the twice larger injection slot size airfoil passes about twice larger jet mass flow rate with the momentum coefficients also nearly doubled. The CFJ airfoil with the largest slot size has the least stall angle of attack(AoA). When the injection slot size is reduced by half, the stall AoA and the maximum lift coefficient is increased. However, when the injection slot size is further reduced by half, the stall AoA is still increased, but the maximum lift coefficient is lower due to the smaller momentum coefficient. The trend of the stall AoA and maximum lift agree with the experiment. At low AoA, both the computed lift and drag agree fairly well with the experiment. At high AoA, both the lift and drag are under-predicted. The reason may be that the RANS model can not handle the turbulence mixing well at high AoA.