Increasing desktop size of Xephyr/gdmflexiserver/Xnest

In OpenSolaris there is a menu entry named "New Login in a Windows" as described
here. This calls the command

gmdflexiserver --xnest
which uses Xnest as the X-Server in the window.

The size of the desktop is relatively small. Even some menus do not show the "ok"
button to quit the dialog.

To increase the size of the desktop in the desktop you click on the desktop of the
Xephyr/Xnest windows and select "Screen Resolution". In this dialog you can change
the size and acknowledge.

The size of the Xnest/Xephyr/gdmflexiserver is changed immediately.