2motion

You can add this into the Additional CSS section under Event settings. This will change the image width on every event page (the height is already set to auto).

.mep-default-feature-image img{
width: 500px !important;
}

If you are getting a 500 Internal Server error, you should contact your shared hosting service provider. It could either be a misalignment in server files (which might be caused due to outdated PHP version) or maybe a memory issue (in that case, only they can fix it for you).