This feature is only available to customers on paid plans. To upgrade, contact your Customer Success Manager or firstname.lastname@example.org.
The max size of a CSV report in Heap varies based on where you download it from in your account. When you download a .csv file from the Users & Sessions view, it will export the most recent 5,000 users who fit the criteria of the query. When you download a .csv file from the Charts view, the export will show up to 100,000 rows of data per metric in your chart. There is no overall file size limit.