Warning: Increasing the export to the maximum amount, 1048576, may cause management of the device to slow when exporting but will not affect the traffic traversing the firewall.
From the GUI:
- Navigate to Device > Setup > Management
- Click the Edit/Gear icon for the Logging and Reporting Setting box and navigate to Log Export and Reporting tab.

- The second option down is Max Rows in CSV Export

- Set the value to the desired number (1 - 1048576).
- Commit change
From CLI:
- Go to configuration mode by entering configure in the CLI
- # set deviceconfig setting management max-rows-in-csv-export <value>
- # commit