I do not know of any limits but it is quite simple to check by setting any large string data to a parameter. As an alternative solution you may serialize you data either through JSON, XML or binary directly to the model stream and just keep a link to the storage within the macro feature parameter. Take a look at SolidWorks 3rd party storage:
In addition the data can be extracted directly from the compound storage (<SW2015) or through the Document Manager (>=SW2015);
The only thing as you need to take care about removing the streams if the macro feature is deleted.