<?php
add_filter('widget_tag_cloud_args','thisbailiwick_set_tag_cloud_font_size');
function thisbailiwick_set_tag_cloud_font_size($args) {
    $args['smallest'] = 12; /* Set the smallest size to 12px */
    $args['largest'] = 19;  /* set the largest size to 19px */
    return $args;
}