Doesnt matter, just be sure to measure 'accurately' preferably using a micrometer or dial indicator caliper taking several measurements (minimum three measurements, more measurements are better) and then averaging the results. 25.40 mm = 1.000 inches.
Usually most cutless bearings will have an embossed stamping on the OD of bearing that will identify and correlate to the actual dimensions ... so try not to totally destroy the old bearing when you remove it.