Leo, I max out on the trees I use at around 10" or so; any more than that I find there’s a transportation problem when I display at shows. Of course, if your efforts are remaining in situ I suppose that doesn’t quite matter so much.
I use the Firs made by Model Scene and very good they are too, in my opinion. See my T-64B from 3 Shock Army - Skif Kit if you’re so inclined. Of course, that represents a young plantation not the density say, of an Ardennes forest but if I manage an effect I’m generally happy.
So, in answer to your question, I would see nothing wrong with scaling them down a little.