You are mixing two aspects of ASME CC181; i.e. calibration and demonstration. CC181 states that you are to follow the rules in Article 4 of Section V. Since CC181 is only addressing the permission to use an alternative acceptance criteria on pipe welds you are therefore required to use the piping calibration requirements. T-434.3 states that the calibration block shall be from a section of pipe of the same nominal size and schedule. So as for the "calibration" portion of your question the answer is no, you are not permitted to use a range.
The issue of demonstration is separate. This is not specified in the ASME CC181. However, EPRI has been for years "qualifying" techniques with a tolerance of about 6mm above and below that demonstrated on. For now, (until ASME defines this parameter) I would think that that should be a reasonable limit. A reasonable range of diameters addressed by the demonstration should also apply. Fig. T-4184.108.40.206 provides a graph of curvatures limits used for curved "plate". Six diameters are identified on that image over the range of 1 to 20 inch diameter. I would suggest that this is a reasonable guide for the demonstration block diameter ranges.