Select the Masterborder of your choice, ()
Draw a text box inside the layout area and select size layout to text! ()
Then select wrap to browser width! ()
Now turn on frames for all borders used, such as top, bottom, left or right. () Every masterborder used, must have the autoframe turned on, or the masterborder must be set to 0 pixel. For Example, if you only need a left masterborder for your navigation, set the width so the buttons fit, then set all other margins to 0 pixel.
If you are using a top frame, type <center> () by clicking the html button on the frame property palette, () beginning of body tab. Set total page width to 600, this way it will look good in all resolutions 640x480 and higher.
See autosize to browser width with frames in action!