Images always thumbnail size

Hi Anna,
first of all, thank you for this great plugin. It is simple and easy to use but efficient.
However I encouter an annoying issue with Gutenberg, even if it is not blocking as I found a workaround. Images in the slider are always of square size. With my theme, it appears to be the thumbnail size which is always used. I can workaround this by converting the slider to a set of images, then change the image size one by one and reconvert the set of images into a slider. Then the size of the images inside the slider is correct.
Is there a way to change the default size of images inside the slider ?
Thanks in advance for your help and keep up this good work ??
Best Regards,
Denis.