Adding custom image size to accordion slider

Hello,

I have a question that seems to have been answered already a few times, though I don’t get it working, so I have to log another topic for this one.

For the Accordion slider I want to use images with an A4 portrait aspect ratio, like 595px × 841px or 300px x 424px. I used the plugin “Simple Image Sizes” to add the 300 x 424 size and regenerate the images, but still it does not show up in the options I can choose from in the Accordion Sider, which shows a small subset of the available options. I also tried adding it to the function.php of my child theme, but it does not show up in the selection. Could you please clarify this once again?

Next; looking at the number of customers needing this feature, it seems like a good plan to add custom formats as a standard functionality out-of-the-box in the theme?

Kind regards,
Tom