I think a micrometer is the best bet as callipers can easily be fooled by positioning it wrong etc. If the cutter has an odd number of flutes then neither will get far...

As they were from eBay the cutters may have been reground, which would make them slightly under the original standard (metric or imperial) dimension. I would start with using the nearest round number (unless it's way off) on something that's not critical, measure the part and compensate from that. It all depends on just how accurate you want to be.