1.Select the edge that you want the model scaled from and hit Shift+S and select from menu Cursor to Selected.
2. Change Pivot Point to 3D Cursor and enable Proportional Editing.
3. Scale the edge and scroll the middle mouse button to increase or decrease the influence of proportional editing.
To to scale only on one axis hit X,Y or Z buttons while scaling, or to exclude one axis use Shift+X,Y or Z
Not a perfect answer, but a good add-on: http://blenderartists.org/forum/showthread.php?335568-Addon-Directly-Set-Length%28edge%29-and-Angle%28edges%29-v0-2