Are there specific dimensions a featured image should have. First i used images with a width of say 500px, and the event calendar resized these so they could fit in the calendar. But it made the images look a bit blurry. Is that supposed to happen? Is the default width of the images between the 280px and 285px or does this depend on which theme and site-width i use?
Do you mean within the tooltips you see when you hover over an event? If so, by default The Events Calendar tries to use a 90x90px version of the featured image here.