Think with a user-focused mindset when creating tags. Proceeding from the example you gave; will there be other articles on similar topics on the site? For instance, if you are going to write other articles about WordPress plugins, but haven’t opened a category with this name, you should create a tag named “wordpress plugins”. If a visitor reading your “WordPress SEO plugins” article wonders, “Are other plugins also introduced on the site?”, they will click on the tag and read the others. However, if you are not going to write another WordPress plugin article, there is no point in using this tag.
Once you grasp the logic behind using tags, it doesn’t matter how many words they consist of.
It is also worth taking a look at this: Should Tags Be Open to Indexing?