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Using "Fullwidth" option for slider

When you choose “Fullwidth” for slider, it makes the container fullwidth, not the image. So, you should try to upload a bigger image or use a plugin like "Regenerate Thumbnails" to resize the uploaded images if they're already big enough. https://wordpress.org/plugins/regenerate-thumbnails/

By the way, don't forget to set a high image width for slider. Navigate to "Appearance > Customize > 4.5. Slide Image Width" and you'll find the necessary information there. You should upload your images into the media library (or use Regenerate Thumbnails) after you set the slide width value. So the process ordering is like that:

1) Choose "Fullwidth" option for slider. (4.3. Position & Style)

2) Set the slide image width. (4.5. Slide Image Width)

3) Upload your images into the media library.

4) Create your slides.

That's all.