An experimental study was conducted to study the effects of geometric size and surface treatment on the fatigue life of fused filament fabrication (FFF) manufactured acrylonitrile butadiene styrene (ABS) parts. Moore rotating-beam fatigue tests were conducted with four different levels of loadings to obtain the S–N curves. Two different sizes (control size and large size) and three different surface treatment methods (as-printed, acetone-treated, and sandpaper polished) were studied. The larger specimens had significantly decreased fatigue life because of a larger volume, and hence a greater probability of defects for crack initiation and propagation, as compared with the control specimen. The acetone-treated specimen had a smooth surface. Its fatigue life, however, decreased significantly because the acetone treatment caused internal damage that weakened the specimen and was reported for the first time. The sandpaper polished specimen also had a smooth surface, but its effect on the fatigue life was insignificant because the extruded filament direction on the specimen surface was parallel to the loading direction. The present results lead to a better understanding of the effects of geometric size and surface treatment on the fatigue performance of FFF specimens. The study also provides important insights for the design of part size and surface treatment of three-dimensional (3D) printed plastic components for fatigue loading end-use applications.