@Kadri: the ground cover is regulated by 5 maps (reeds, grass, some more bushy areas, the peaty area where the cows are, and sand). The latter needs to be the most precise to get the little trodden paths, so is 8000x8000px greyscale TIF. The others I reduced to 50% or 25%. In total I had 14 maps, the rest for the different trees and shrub, and the water. The water was an overall mask, reduced, for the distant river, and a cropped, but larger one for the closer pools and streams (it takes some calculating where to put them, so they overlap ok). Also 4 painted shaders to get the mud and the reeds in the front pool, and the cows and sheep in there respective places. The buildings were single objects, just put in place. Most maps where around 14-20MB, some 50MB.
I had TG crash on the first try to get the 5200x3250px image rendered, but lowered the cache, and that worked. 2.86GB used.
It would be good to have a way to reduce the mask size, and still keep the black-white(-grey) a precise as needed (bitmaps are not accepted, alas). It is of course possible to crop the masks, as large areas are black, but that takes more calculating to fit them in.
---Dune