There are two ways to use an Arnold area light in MtoA. You can either add a regular Maya Area Light, in which case Arnold will assume a rectangle/quad source or if you require a different shape, you can alter the light node type to be an Ai Area Light.
Note that Arnold will render assuming that the area light is a Quad. If you need a different shape, you will need to select Ai Area Light as the node type instead. The Ai Area Light is a custom Arnold light that enables the user to create realistic lighting effects based on different preset shapes such as Disk and Cylinder (as well as Quad).
The most important attribute of the Arnold Area Light is 'Light Shape'. This specifies whether the area light is a cylinder shape, a disk shape, or a quad (rectangular) shape. This is the first attribute you should set - choose the one which matches the geometry to which you are attaching the light.
Detailed explanations of the other controls are described in the following topics, which show examples of using the Arnold Area Light with each of the Light Shape types.
Different area light shapes visible in the specular reflection of a sphere
Note that if you just use a regular Maya Area Light instead of the Arnold Ai Area Light, you will have all the control attributes under the Arnold tab that you would for an Ai Area Light if you selected the Quad shape (apart from the 'Light Shape' attribute). See the Quad Light Shape topic for details of how the Arnold Ai Area Light works with the Quad shape selected.
The Lights page has more detail about the controls. In addition to honoring the standard Maya light attributes, the Attribute Editor will also show the following attributes under the Arnold group:
Ai Area Light attributes