All types of mirrors may be used for scrying and the size is not important. Because mirrors are linked to the moon mirrors should be backed with silver. Try and use a round or oval mirror instead of a square mirror. For the frame try and use a mirror that has a silver frame. Old mirrors also seem to work better than new mirrors.
Most seers prefer to use a black mirror.
Because this is difficult to buy you may have to make one. Just simply take out the glass and paint it black. You may have to give it a few coats of paint though. When you put it back in the frame make sure the glass part is to the front. The use of black mirrors may be traced back over the centuries.