The ferromagnetic-paramagnetic phase transition in an Fe monolayer grown on a 2 ML Au/W(110) substrate and capped with a 1 ML Au layer was studied with spin polarized low electron energy microscopy. When the data are analyzed as in previous studies, not only the Curie temperature T(C) but also the effective critical exponent beta depends upon the terrace width. Taking finite size effects into account gives the terrace width-independent two-dimensional Ising model value beta=1/8.