IIRC the protected range of a transmitter is specified on the WT Act licence and/or ANO approval for the service being provided. For most comms services and navaids the DOC is usually shown in the AIP but this does not seem to be the case for ATIS. I think you will find each is different - partly because a smaller DOC may have to be accepted to get the frequency assignment. I suspect you'll be surprised at how small some are - at one international airport I worked at I think it was 15000ft and 25 miles although most crews would be able to report receiving it at 100+ miles out. All this is for an arrival ATIS - for a departure ATIS it's typically surface and 2 miles.