It seems that you do not want any scrolling and want to use the whole screen available.
This can be done defining at least two positioned div's in your style sheet (you have to adjust the numbers for left,top, right and bottom/height for your needs !):
#nachricht {position:fixed; left:180px; top:120px; right:40px; bottom:250px; overflow:auto;}
#eingabe { position:fixed; left:180px; bottom:0px; right:40px; height:240px;}
This way you can have your windows scaled depending of the screen format. The div #eingabe will always use the bottom 240 pixel of the screen, the window #nachricht will use the width (between left and right) of the screen with a variable hight (starting an 120 pixels from top down to 250 pixels from bottom) if the content in the #nachricht window does not fit, a scrollbar will appear on the border of it.
