IMO, avoid using surfaces as principle scene light sources. I know people have been talking about it here, because other PBR renderers use them heavily. But nVidia recommends against it in their documentation, for performance reasons, and they provide plenty of other native objects that are specifically optimized as light sources. Save the emissive setting for when you need an object to light up (TV screen, sci-fi control panel, etc.), not to provide the lighting for your scene.
For future reference, to make an object emissive, select its surface in the Surfaces tab. Go to the Uber Iray shaders in your Content folder, and find the one that says Emissive. Apply it. This zeros out all the other settings (very important) and "activates" the emissive property simply by changing its emission color from all-black. You can dial in the color you want, and specify the strength of the light.
If you want a broad light source, use a spot light, and change its emitter from Point to Disc or Rectangle. Then alter the width and height of the emitter. You can make gigantic light panels this way -- the ,measurements are in centimeters, so a 200 by 200 emitter is over six feet square (or diameter). Iray is more efficient when using its internally-defined light sources. Two other benefits: 1) you can view the scene "through" the light without the use of auxilliary cameras, and 2) you can very easily hide the emitter from the renderer by selecting this property for the spotlight.
