Generally, the width of the picture light should be at least half that of the frame you are lighting. For instance, choose a picture light with a width of 10 inches if your frame is 20 inches wide. The width of the picture light also depends on the frame layout. For example -
i) Choose a picture light for the portrait frame that is roughly half the width of the image.
ii) Select a picture light for a landscape frame that is between one-third and one-half the width of the image.