sets-size-of-bp-cover-image.php

<?php
/**
 * Sets the size of buddypress profile cover image.
 *
 * @param int    $size size in bytes.
 * @param string $type upload type(avatar, cover_upload).
 *
 * @return float|int
 */
function wbcom_sets_size_of_bp_cover_image( $size, $type ) {

	if ( 'cover_image' === $type ) {
		$size = 50 * 1024; // change size with your own.
	}

	return $size;
}

add_filter( 'bp_attachments_get_max_upload_file_size', 'wbcom_sets_size_of_bp_cover_image', 10, 2 );