I know that I can design corner windows by using one of the two freely-constructable windows and check Miter left or Miter right in the suitable dialog box.  I would now like a trapezoidal porch with windows in such a way to disguise the fact that all the windows are pushed directly together. From where do I learn how long the window of the medium side of the porch should be, so that it fills out the whole side? 

Use as the default window width a dimension which is longer than the wall, and check both Miter left and Miter right. If you now position the mouse into the relevant wall, the window is automatically set to the correct length.