I'm not sure there is a solution to this, yet.
As you can not create lights in Scene Nodes, yet, you'd have to get the information out into classic C4D somehow.
I tried to use Xpresso to access the generated Selection tag and from there read the polygon information, get the points and calculate the size. But I'm not able to get Xpresso to read anything useful from the generated Selection tag. Not sure, why. I tried priorities and object order in OM, nothing resulted in Xpresso successfully iterating the Selection tag (selection count is always shown as zero).
This does not mean, this Xpresso approach does not work, I may simply be too stupid to get it to work.
Another approach could be to use a Python tag to try similar. But currently I lack the time to do further investigation.
Sorry!