For most of our examples on our demo, we simply added a div around the [intense_counter] shortcode that set the font-size in the style tag. You could use the [intense_heading] shortcode as well, then place the [intense_counter] shortcode inside of it.
Here is an example from our Counter demo page.
<div style="font-size: 90px; line-height: 90px;">[intense_counter font_color="primary" speed="30"]</div>
If you need any additional help, please let us know.