It is possible to change the number of rows displayed in email reports, but currently this is a global change and so it is not possible to customise the number of rows in individual reports (see feature request).

To change number of rows from the default of 23 rows to for example 50 rows, please edit your config/config.ini.php file and and set under the [General] category:

scheduled_reports_truncate=50

All reports in your scheduled email reports will now list up to 50 rows.