Alison,
I can help you out with this. I took a look at your site and found the theme is limited the entry-content div to 604px. This is done by your theme and not the plugin.
If you add this coding to your stylesheet on that page only it will make the calendar full width.
.events-archive .post-0 .entry-content {
max-width: 1080px;
}
It is possible to take the header of the site off the calendar page, but that would require customizing your theme.
If you are looking to modify the filter bar above the calendar please take a look at our Themer’s Guide on how to do that.
https://theeventscalendar.com/support/documentation/events-calendar-themers-guide/
Let me know if you need any more help.
Thanks