I guess it would matter most of how it looks with your home? I would think length and size would be desirable to be complementary with the window size and orientation. I have seen many boxes made that actually were made to hold a existing planters so the plants and container could be removed for easier care. Find your inside containers first if you chose to use them so you can build the box to fit what ever you chose to put the plants in, the 8" will give enough space for most plants and flowers but I would say have a helper hold a board or carboard cutout up to the window and step back and see what will look better before building it. If it a large window a taller box may look better?