You cannot limit the length of any attribute in a "Reporting Datasource". As the name suggests, it is a Data 'Source'.
If you want to limit the length of any attribute, you need to handle that in the process context data (design-time).
If your process context attribute consists of values which are exceeding the standard size of a string attribute, concat (use built-in java function : 'substring()') that value and then pass it into a context data attribute before using a Reporting Datasource activity.
It is simple. Please do not complicate the situation.