Specific heat dissipation figures for PCB ground plane heatsinks in switching regulators?

I'm designing some switching power supplies and the regulator ICs connect to the ground plane with multiple vias. The customer wants the designs as small as possible.

Using the ground plane as a heat sink seems common but I can find no specific information. I have seen 10 sq.in. of ground plane specified for a part and two statements saying that anything over 1 sq.in. is ineffective, but no specific information.

Most of what I've seems vague or anecdotal.

Are there any specific figures for heat dissipation for ground planes as heat sinks? I have also seen thermal images showing the ermperatures on the ground plane around an IC heat source but can't find them now. Any information or links would be appreciated.

Here is what I would suggest and you can NOT, repeat, you can NOT go wrong with this approach. I appreciate where you are coming from. Worked with many of the best board design & simulation folks out there, but it seems they still don't have a good handle on PCB as an electrical/thermal device, specifically Mentor Graphics.

So here is how you can't go wrong. Design a board (you decide) with some heat generating device package on it. Hook it up run your own thermal test and determine the limits of what you are up against.

It doesn't have to be an all out board design, with the idea in mind you are testing the thermal characteristics of what you intend to use. You could do the board for less than $50, and in less than a week.

If I had to succeed with little or no information, other then hear say, this is what I would do, This is what I have done.

Again when Mentor Graphics can't even tell you what you need to know, then you have to do it your self! Do a PCB thermal model.