What is ideal size for Thumbnails in an Event

I noticed that when I uploaded an image for a thumbnail, although in the editor it displays in full, when I view it in mobile version, the image is cropped. I see no tool to ensure it displays on both platforms. So, what are the ideal dimensions for an image to display on both platforms?

Can you share link to event page?

Here 'tis and thanks for the help

https://taystthenotes.com/upcoming-events

9 hours ago, tuanphan said:

You can use this to Website Tools (under Not Linked) > Custom CSS to make image fullsize on mobile

 

Thank you tuanphan