@Patrick_Maroney the CSV export creates a single document that is then stored in Elasticsearch, so the maximum is dependent on what Elasticsearch itself can support. There's http.max_content_length setting discussed here that you'l have to keep the xpack.reporting.csv.maxSizeByte under or it'll error out when inserting the document into Elasticsearch.
With that being said, making this value too much larger than the default 10 mB limit is untested and unsupported, and this should be done at your own risk and it has the potential to negatively affect the performance of your Elasticsearch cluster.