If the light fixture is above the mirror, just measure the distance from the fixture to the mirror. If the light fixture is built into the mirror, measure from the inside edge of the mirror to the fixture and verify the width of the frame will fit in that space. If the light fixture hangs directly over the mirror, measure the distance between the light fixture and the face of the mirror. Be sure to choose a frame style that is thin enough to lie between them. Not sure the frame will fit? Buy a sample for $1 and test it yourself!