CSV and XLSX exports are limited to 1 million, so you can only export 1 million rows of (contact) data at a time.
There is a notification set on the export button which will remind you of this.
If you do attempt to download a file with more than 1 million rows then it will simply limit at 1 million.
If you do need to download a CSV or XLSX file with more than 1 million rows please attempt to use a workaround (i.e. creating a series of smaller lists, downloading and combining.)
However, if this is essential we are able to export it for you via a support ticket. It will take a day of engineering time to do this, however, delivery timing depends on support capacity; when the ticket comes in we will provide an estimate.
Note - if you attempt to split a segment with 1 million or more contacts this will also cause issues and crash your usage of Ometria.