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I found the answer, and it's what I expected and wanted. Maxlength applies to each item in a list, so the following works:
<cfqueryparam list="true" value="1,2,3,4,5,6" maxlength="1" />
whereas, the following throws an exception:
<cfqueryparam list="true" value="1,2,3,4,5,26" maxlength="1" />
Exception:
"The cause of this output exception was that: coldfusion.tagext.sql.QueryParamTag$InvalidDataException: Invalid data value 26 exceeds maxlength setting 1.. "