How to crop cover image

BuddyPress does not offer cropping for the cover image at present. As a website admin, you can define cover image size using custom code.

For best results, upload an image that is 1024px by 300px or larger.

function your_theme_xprofile_cover_image( $settings = array() ) {
   $settings['width'] = 1170;
   $settings['height'] = 250;

return $settings;
}
add_filter( 'bp_before_members_cover_image_settings_parse_args', 'your_theme_xprofile_cover_image', 10, 1 );

Ref: https://codex.buddypress.org/themes/buddypress-cover-images/