How can I change the PCB board size in Eagle .brd file, so that I can construct large boards ?
Is it a proper way to use 'move' tool to adjust corners, thus can increase board size ?
The board area is defined by the 'dimension' layer - 20 I think. Either drag it, as already suggested or delete it & draw it. Be careful to set the grid before drawing it if you want an exact size. For example, if you want a 100x160mm board you don't want the grid set to 0.1".