WordPress creates duplicate images in different sizes upon upload. These sizes can be set via Settings > Media.
By default the “post-thumbnail” size of your theme is being used as source for the featured image.
And the maximum width of the featured image is by default 40% of the event info area.
You can change the featured image size and maximum width via Settings > VSEL. Use both settings together, in order to get the size you want.
The featured image on the single event page is handled by your theme.