This paper proposes and demonstrates a simple method using the intensity correlation of X-ray fluorescence to evaluate the focused beam size of an X-ray free-electron laser (XFEL). This method was applied to the sub-micrometre focused XFEL beam at the SPring-8 Angstrom Compact Free Electron Laser, and the beam size evaluated using the proposed method was consistent with that measured using the knife-edge scan method. The proposed method is readily applicable to extremely small X-ray spots and can be applied for the precise diagnostics of sub-10 nm focused X-ray beams which have recently emerged.