Sigh. The case is much more complicated.
Consider drop shadows, tables that exceed the bounds of text frames, etc.etc.
But you could export the whole group as image, place that same image at 100% scale and measure after placing.
Still, with that method there will be unnecessary "white space" or transparent pixels in your exported image. To get the net size of all writing pixels you have to use cross scripting with e.g. PhotoShop.
Uwe