Excerpt lengths

WordPress post excerpt length:

function custom_excerpt_length( $length ) {
	return 20;
}
add_filter( 'excerpt_length', 'custom_excerpt_length', 999 );

Buddypress Activity stream excerpt length:

function cc_custom_excerpt_length() {
return '500';
}
add_filter( 'bp_activity_excerpt_length', 'cc_custom_excerpt_length' );

via Plugin API/Filter Reference/excerpt length « WordPress Codex.