X-windows and fvwm2?

["Reza Entezari" <entezari@hotmail.com>]

I have just installed Debian and X-windows which has
a few window managers like fvwm2 and WindowMaker and ....

But, the viewport is not properly set. The viewport is smaller
than the size of my desktop;

How can I adjust the viewport in X/WM ?

Any help would be appreciated,
-Reza