By default WindowSizer tiles all open non-minimized top-level application windows. To quickly create a more selective arrangement use the "Tile Last" command specify the exact number of your recently used windows to tile. Also available as a keyboard shortcut of Ctrl-Alt-[1..9].
You may also place the windows you wish to tile in the foreground and choose the "Tile Topmost" command. The topmost windows "pop" into position. All partially obscured windows are excluded from the arrangement in the "Tile Topmost" command.