The effects of graphene oxide (GO) size and reduction method on mechanical property and electrical conductivity of GO papers are studied. GO sheets are prepared using the modified Hummers method and GO papers are fabricated through vacuum filtration of GO aqueous dispersions. To study the size effect, the as-prepared GO dispersion is sorted into four different groups with uniform sizes after repeated centrifugation and collection of supernatant. It is found that the Young's modulus, tensile strength and electrical conductivity increase consistently with increasing the GO size. Three methods, including hydrazine reduction, hydrogen iodide reduction and thermal treatment, are employed to reduce the GO sheets. The highest electrical conductivity of 139,000 S/m is obtained after thermal reduction. The mechanisms responsible for these observations are discussed based on the elemental, structural and morphological analyses of GO and reduced GO sheets.