Go ahead and open /wp-content/plugins/all-in-one-wp-migration/constants.php
Find these lines:
// = Max File Size =
define( 'AI1WM_MAX_FILE_SIZE', 536870912 );
You’ll see the max size is defined in Bytes. In order to increase the upload size limit, simply add the number you need.
You cannot set a value, that is higher than your hosting plan's max upload file size (upload_max_filesize). To increase this value, you can upgrade your hosting plan