Find this function in the front-page.php file of your theme: <?php bb_tag_heat_map(); ?>
And replace it with this function: <?php bb_tag_heat_map(9, 9, 'pt', 80); ?>
Here’s what those numbers mean. The first number is the minimum font size, the second is the maximum font size, the third is how to measure the font (pt, px, etc.), and the fourth is how many tags to display.