#858549 might be a dupe of this one. As I noted there[1], it turns out that instead of resizing the display directly, spice-vdagent now just informs Gnome that the resize is needed. Consequently, resize only works with Gnome.
(I found that information in a related redhat ticket, [2].) -nd. [1] - https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=858549#15 [2] - https://bugzilla.redhat.com/show_bug.cgi?id=1290586
