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 It could either be a misalignment in server files (which might be caused due to outdated PHP version) or maybe a memory issue (in that case, only they can fix it for you).
