bp_media_file_upload_max_size()

function bp_media_file_upload_max_size( $post_string = false ) {
	static $max_size = - 1;

	if ( $max_size < 0 ) {
		// Start with post_max_size.
		$size = @ini_get( 'post_max_size' );
		$unit = preg_replace( '/[^bkmgtpezy]/i', '', $size ); // Remove the non-unit characters from the size.
		$size = preg_replace( '/[^0-9\.]/', '', $size ); // Remove the non-numeric characters from the size.
		if ( $unit ) {
			$post_max_size = round( $size * pow( 1024, stripos( 'bkmgtpezy', $unit[0] ) ) );
		} else {
			$post_max_size = round( $size );
		}

		if ( $post_max_size > 0 ) {
			$max_size = $post_max_size;
		}

		// If upload_max_size is less, then reduce. Except if upload_max_size is
		// zero, which indicates no limit.
		$size = @ini_get( 'upload_max_filesize' );
		$unit = preg_replace( '/[^bkmgtpezy]/i', '', $size ); // Remove the non-unit characters from the size.
		$size = preg_replace( '/[^0-9\.]/', '', $size ); // Remove the non-numeric characters from the size.
		if ( $unit ) {
			$upload_max = round( $size * pow( 1024, stripos( 'bkmgtpezy', $unit[0] ) ) );
		} else {
			$upload_max = round( $size );
		}
		if ( $upload_max > 0 && $upload_max < $max_size ) {
			$max_size = $upload_max;
		}
	}

	return bp_media_format_size_units( $max_size, $post_string );
}