Learning how to use Joomla's Module Class Suffix will allow to customize certain modules without editing their code.

However, you will need to inspect a little into your website's template and you require some CSS knowledge

The Module Class Suffix is a parameter every Joomla module should have, and it looks like this:
Module Class Suffix  

But before you apply it to your module, you might want to view the source code of your live module in order to understand the difference between before and after. I use either one of the following Firefox Web Developer Tools to do this:

• View Source
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• Outline -> Current Element
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Templates usually write different HTML arround the modules depending also on the position you publish them (Module Chrome). For example, let's say your target module's code looks originally like this:

<div class="module">
	<h3>Title</h3>
	{Module Content}
</div>

<div class="modulecustom">
	<h3>Title</h3>
	{Module Content}
</div>

.module {
	padding: 10px;
	...
}

.modulecustom {
	padding: 10px;
	...
	font-size: 150%;
}

So every module wearing the custom class suffix will now have a certain different font size.