I was sitting here thinking about all the amazing projects I've done over my career, and the most common thing I hear with working on custom spaces us that they wish they would have went a little larger on the custom piece. I get it. Once you have it in the space, or your frame in the window area , it will be smaller to the objects around. So, when you are planning your space, just think about how large you want it and maybe add 20%.
Last advise for the day. Think about detail, what objects beside the major focus do you want in the art work?