The configured row limit for scheduled email reports is here: https://github.com/matomo-org/matomo/blob/4.x-dev/config/global.ini.php#L560

However, it seems that this limit is not used when sending or downloading the report in the Matomo UI - this seems to be limited to 100 rows and is not changed by increasing the config limit linked above.

This can be reproduced by creating a scheduled report for any report that has more than 100 rows, increasing the config.ini.php to more than 100 and sending a report using both methods.
The one sent by the scheduled task will correctly use the scheduled_reports_truncate limit and the one sent by clicking the button in the UI will be limited to 100 rows.