So ...

We are going to use a handy plugin called Regenerate Thumbnails.

Here it is:

https://wordpress.org/plugins/regenerate-thumbnails/

It's a very small plugin (74 KB), but let's see the usual speed tests ...

Here are my results before installing the plugin ...

... and the result after installing and activating the plugin:

So it's a big plugin, but is definitely a well-coded one ... No more comments needed ...

Now let's see how it works ...