Slider tutorial - What's the best responsive way to set up a showcase slider?

The best responsive way to set up a showcase slider is to set the slide size and slider size the same values, and set the maximum slider height to the slide's and slider's height.

Let's see an example with images! I'll be using a slider which has 300x400px size, but you can use any size you want.
1

Set the slider size

Go to the Slider settings → Size tab and set your slider size to be 300x400px.

2

Set the slide size

Go to the Slider settings → General tab and scroll down to find the Showcase slider type section. Set the Slide size to your slider size, which is 300x400px.

3

Set the slider's responsive mode

Go to the Slider settings → Size tab, scroll down and select the Full width responsive mode. Set the maximum slider height to the slider's and slide's height: 400px.

You can use the Auto responsive mode as well, just make sure the Upscale is turned on!

Configure the slider to your liking and add your content. 

If you check the slider in your browser, you will see that the slides fill the whole browser. And when you reduce the browser width, the slides won't start shrinking but you will see less of them.