Ironic: https://mail.gnome.org/archives/commits-list/2012-December/msg00170.html

"Added constraint to minimum window size.

In fair play I should implement get_preferred_width/width_for_height and friends methods on the views, but since I'm lazy and these days I doubt someone using calendar will have less than 800x600 pixels."

Looks like they still use that size to this day.