How to set the default size for a Xephyr window?

[Markus Schönhaber]

Hello,

If I select
Applications / System Tools / New Login in a Window
Xephyr is started and opens a window with the login screen.
This window is too big to fit completely on the screen. How do I change
it's initial size?

I can, of course, change the resolution after logging in but it'd like
the Xephyr window to come up with a more appropriate size.

Regards
  mks