check_upload_size

function check_upload_size( $file ) {
	if ( get_site_option( 'upload_space_check_disabled' ) ) {
		return $file;
	}

	if ( '0' != $file['error'] ) { // There's already an error.
		return $file;
	}

	if ( defined( 'WP_IMPORTING' ) ) {
		return $file;
	}

	$space_left = get_upload_space_available();

	$file_size = filesize( $file['tmp_name'] );
	if ( $space_left < $file_size ) {
		/* translators: %s: Required disk space in kilobytes. */
		$file['error'] = sprintf( __( 'Not enough space to upload. %s KB needed.' ), number_format( ( $file_size - $space_left ) / KB_IN_BYTES ) );
	}

	if ( $file_size > ( KB_IN_BYTES * get_site_option( 'fileupload_maxk', 1500 ) ) ) {
		/* translators: %s: Maximum allowed file size in kilobytes. */
		$file['error'] = sprintf( __( 'This file is too big. Files must be less than %s KB in size.' ), get_site_option( 'fileupload_maxk', 1500 ) );
	}

	if ( upload_is_user_over_quota( false ) ) {
		$file['error'] = __( 'You have used your space quota. Please delete files before uploading.' );
	}

	if ( '0' != $file['error'] && ! isset( $_POST['html-upload'] ) && ! wp_doing_ajax() ) {
		wp_die( $file['error'] . ' <a href="javascript:history.go(-1)">' . __( 'Back' ) . '</a>' );
	}

	return $file;
}