Question 1

Alternative 1: Use a Third-Party Tag Cloud Plugin

Accepted Answer

POSSIBLE APPLICATIONS

Display your tags grouped by language or by topic.
Create an alphabetical index of your tags – think of a phone book for your tags.
Display only specific tags in your tag cloud.
Choose which tags to display in different sections of your blog.
Change the sorting order in your tag cloud.
Customize the links, the text, the font size or the separator of your tag cloud items.
Prepend or append to each tag a custom character or the post count.
Insert into each post a tag cloud using only the tags of this post.
Easily manage huge amounts of tags or posts in the backend by dividing them into groups.
Add a parent level to tags and make them hierarchical.

The smallest and largest font size settings for the Tag Group plugin's Gutenberg block

This free plugins provides a number of advanced features for designing custom Tag Clouds in WordPress. I think this the best choice if you don't want to touch any codes. You can easily adjust the font sizes of the tags with its Gutenberg block.

Nonetheless, I like adding custom codes rather than using this plugin. I don't want to install too many plugins, as it will slow my WordPress and increase the risk of having security vulnerabilities. It's overwhelming to use a feature-rich plugin, if you just simply need to change the font sizes. I only recommend it if you're looking for superior and specially designed tag clouds.

Question 2

Alternative 2: Create a Custom Tag Cloud with PHP Code Snippet

Accepted Answer

You can also manually add a tag cloud to your WordPress using the wp_tag_cloud() function directly. You can insert the PHP snippet as a shortcode. It is perfect for creating a custom tag cloud page or for adding them to your theme layout. The official function has already contained many options, including sizes, orders, and formats. It is quite flexible to create custom tag clouds without adding a filter hook.

Default Usage

Example: Cloud limited in size and ordered by count rather than name

Question 3

How to Add PHP Code Snippet to WordPress

Accepted Answer

If you don't know how to add PHP code to WordPress posts or pages, I recommend using the following plugin to make PHP snippets as custom shortcodes.

Gutenberg Block

3 Ways to Change Font Size of Gutenberg Tag Cloud in WordPress (PHP, CSS, JS)