Let me guess: OP, you're not using Gnome, right? According to a related redhat bug[1], there was a deliberate change in spice-vdagentd: instead of resizing the display directly, it simply emits some kind of notification, which only Gnome handles.
Of course, it turns out that resizing does work perfectly fine in a different stretch guest using Gnome and gdm3. With Xfce (and lightdm), automatic resizing doesn't work (except at logout), but it is always possible to force a resize using this command: xrandr --output Virtual-0 --auto Does that sound like the same thing you're seeing? -nd. [1] - https://bugzilla.redhat.com/show_bug.cgi?id=1290586
