There is a display bug in how Zimbra displays shared calendars via web page.
If you visit a calendar's URL (those visible by the public), and resize the browser window, the page resizes successfully horizontally but stays at a fixed width vertically.
This is a problem that becomes especially relevant when you are trying to embed the calendar in an iframe.
If you look at a public Google calendar via an iframe or web URL you will see that the calendar successfully resizes both horizontally and vertically.
This should be a minor code change.