Even broader: any resolution you want. I genuinely have always wondered what is at the bottom of this problem. Why is it that on an otherwise so versatile and configurable OS it is seemingly so hard to change the resolution when logged in remotely? Is it something in X that makes it like that? Was it originally never meant for headless use? Having used, and cursed the fixed resolution of, VNC for years I still remember the first time I used rdp on Windows, my jaw was like on the floor. That is how it should be: logging in without being bound to a hardware screen, and in a seperate session (latter needs a fix though since the default client on non-server windows doesn't allow it)
[0] https://wiki.archlinux.org/index.php/xrandr#Using_xrandr_wit...
We have this implemented, but the problem is that the current version of Xvfb doesn't support randr extension. See https://bugs.freedesktop.org/show_bug.cgi?id=26391 . With the patch attached to that bug CRD should resize desktop automatically.