hope that helps
I don't recall setting them explicitly when I first started coding with XNA and I never got a zero sized screen.
ZMan what exactly makes this a bug?
According to what I know the back buffer height and width properties are only needed to be set if you want to control the resolution.
Now if for example the back buffer value really was zero then you wouldn't be able to see anything because the ziewing space (if you will) is 0, 0.
So this is one of the two
1) It is a bug and the back buffer properties are really suppose to return the current back buffer size even if you haven't set them
OR
2) The back buffer is 0, 0 until you set them then it sends the command to the graphics card to change the resolution to your desired value
In it is good practice to set the back buffers even if it has a default value it will give your application more control. But it is required of you to set the back buffers before your application can go fullscreen.
A 13 Year Old Programmer