Tag Clouds

a filter for the tag cloud widget args;

example:

add_filter('widget_tag_cloud_args','set_tag_cloud_widget_args');
function set_tag_cloud_widget_args($args) {
$args = array(
'number'    => 100,
'largest'    => 25
);
return $args;
}

https://codex.wordpress.org/Function_Reference/wp_tag_cloud

or use one of the ‘configurable tag cloud’ plugin widgets: http://wordpress.org/extend/plugins/search.php?q=configurable+tag+cloud