The thickness dependence of the magneto-elastic coupling B₁, the intrinsic film stress, and the magnetic in-plane ansiotropy K₄ of Fe films on W(100) are measured with an in situ combination of a highly sensitive optical deflection technique with magneto-optical Kerr-effect measurements. We find that both B₁ and K₄ depend strongly on the Fe film thickness. The thickness dependence of B₁ can be described by considering a second order magneto-elastic coupling constant D=1 GJ/m³ as a strain dependent correction of B₁. We tentatively ascribe the deviation of K₄ from its bulk value to the tetragonal lattice distortion caused by an effective tensile in-plane strain of 5.3% in the pseudomorphic region and of 2% in thicker films.