Hi,
we have no dedicated function to retrieve the dimension of a specific display on a GPU. Anyhow you can get this information by enumerating the display devices. For each display that is mapped you will be able to query the origin and the dimension. You can either use ADL or use windows functions like enumDisplayDevices.
I attached you some sample code that actually enumerates all devices using ADL and that provides you functions to get the Displays per GPU. You could use it as below:
enumDisplays();
// Get first display on GPU 0
uiDsp = getDisplayOnGPU(0, 0);
getOrigin(uiDsp, Origin_X, origin_Y);
getSize(uiDsp, w, h);
// Get second display on GPU 0
uiDsp = getDisplayOnGPU(0, 1);
...
To compile the attached code you need to get ADL from: http://developer.amd.com/tools-and-sdks/graphics-development/display-library-adl-sdk/
Chris