Can I create a window larger than the monitor resolution?

I have a monitor with max resolution 16001050, but I want to output 19201080 images.
I have tried the example offscreen.c, but I can only get a 1684*1032 image.

If you want to render offscreen at a specific size I would recommend you use framebuffer objects. The window manager can always override any window size you request, but has no say over framebuffer objects.