How to make the button small?

Hi Guys,

The plugin uses my theme default button size medium size.

<p>
	<input type="submit" value="Subscribe" />
</p>

I tried to add class-button but it didn’t work too.

<p>
	<input type="submit"
      class="button button-primary"
      value="Subscribe" />
</p>

What should I modify to make it use small button instead of the medium?

Thanks!

https://www.ads-software.com/plugins/mailchimp-for-wp/