width and height (in X and Y) of an image in 3D

how do you find out the width and height (in X and Y) of an image
after you've added a 3d camera?

I know the camera in Z is 782.21 by default.

Select the image, press Shift-F to reveal the source media in the Media tab in both the Layers tab and the Inspector. In the Inspector, you'll see the dimensions.