He spelled it right
As I said I intend to insert into Word and print after that. So I have calculated that an A6 sheet size will be (in average) the best size I could handle.
Well, if A6 is your desired size, my option isn't really viable.
But it seems one needs to make it in each original image, because when there are a plenty of images in layers, "Image > Scale image" will affect all layers, not only the selected layer.
Try "Layer > Layer Size"
Lets say that original image 1 and 2 have 72 ppi. Then I create new image in Gimp with 300 ppi and I insert the images as in my former example. Should I reconfigure both images to 300 ppi before using/inserting them? Or once new Gimp image is configured with 300 ppi will the others automatically take this main configuration when printing?
Hmmm.. I'm really not sure but I think the first layer (the new image you created in Gimp) defines the PPI, so the new image would also have 300ppi in the end.
In an image created using layer structure (as my example), is there any way to handle the image by clicking over it in screen? Or is just through clicking over layer the way to select the image? It is because with many layers to find the layer is more difficult than just click on the image parts in screen.
I don't really get your point here to be honest. If you click with the scale-tool in the main window (the one with the menu on top), the active layer will be scaled. When using the move tool, you can choose whether the thing you click on or the active layer should be moved by going to the tool options.
But again, I think I didn't get your question there.