You have to edit ‘template-functions.php’ which is located in the bb-includes folder. The function is at line 890, called tag_heat_map. You will see that it takes 4 variables as parameters the first one is the smallest size possible for the tags, the second one is the largest size possible for the tags, the third is wether it the font size is measured in points(pt), pixels(px), or ems(em). The default is pt. The last parameter is the maximum numbers of tags that are displayed.