It’s a little hard to tell with a link when you’ve done something custom. However, my suggestion would be to set a width limit on the side bar or if you have one then look using CSS to decrease that font size – one thing has to go down or up to fit from what I’m seeing 
Thanks. The only thing I have customised that I think would affect this is reducing the width of the sidebar to 198px from 224px. For some reason the tag cloud generated by sidebar.php isn’t respecting the CSS width of the sidebar. I am not sure how the font-sizes for tags are generated. They are set using an inline style tag, so there is no CSS class to override.
See if this goes to your convenience.
Forum Tags
Add this filter to your child theme function.php
Cloud font values are set by default to 10 and 42.Seted to 10 and 30
`remove_filter (‘bp_forums_tag_heat_map’,'bp_forums_tag_heat_map’);
function my_bp_forums_tag_heat_map( $args = ” ) {
$defaults = array(
‘smallest’ => ’10′,
‘largest’ => ’30′,
‘sizing’ => ‘px’,
‘limit’ => ’30′
);
$r = wp_parse_args( $args, $defaults );
extract( $r, EXTR_SKIP );
bb_tag_heat_map( $smallest, $largest, $sizing, $limit );
}
add_filter (‘bp_forums_tag_heat_map’,'my_bp_forums_tag_heat_map’);`
Also in functions.php
WP Tag Cloud defined in wp-includes/category-template.php
`add_filter( ‘widget_tag_cloud_args’, my_tag_cloud_args );
function my_tag_cloud_args($in){
return ‘smallest=10&largest=12&number=25&orderby=name&unit=px’;
}`