Tell us what you are trying to do?
Users upload images into a repeatable image field, then I display them in an image slider that is within a view. Right now I am only able to use the Original image placed inside the slider, but some images are very large and take a long time to load... Is there a way to display one of the thumbnail sizes instead to cut down on the image load time?
Is there any documentation that you are following?
There isn't such an option available, to specify the source image size used in the Image Slider block.
Two of the three slider styles use a large image (either directly, or in a lightbox), and only one uses just smaller images.
WordPress 5.3 introduced a new way of handling very large images for the use case you describe, where users submit photos (e.g. from phones) that are far larger than needed, and it produces a maximum size (default 2560px) to be used in place of the original version. But in the slider (where you cannot specify the size) we still use the original version.
So I think we can make some improvements here. I'm going to raise this as a feature improvement.
I don't have any suggestions to help you right now, though, I'm afraid, but I'll push for this to be improved in the upcoming dev cycles that focuses on usability improvements.
