I had been speaking quite a bit to a client over the past few days. He has a huge wall and knows exactly what photo mural he wants: the beautiful Notre Dame Gargoyle overlooking Paris on a misty morning.

The issue we discussed at length was placement on the wall. He thought about using a smaller size of the image. Covering the entire wall was not in his decorating budget. Eventually he decided that just because the photo mural comes as a wallcovering doesn't mean he has to cover the entire wall! For his large wall, this client eventually decided to order his mural to fit the height of the wall, but not the width. He intends to center the mural.
I have had the same experience myself: a very large wall that even the largest version of the mural I chose wouldn't cover the entire wall, which was what I had originally intended. Guess what? I love the result. The room was still transformed into a Mediterranean villa (this was my personal photo mural choice), and I now have wall space on either side of my mural. If I want to put a chair or table against the wall, I won't be covering a part of my mural. That ended up being important to the functionality of my room, as furniture gets moved around from time to time.
It's pretty standard to measure the entire wall and then order your mural in that exact size. Sometimes, covering an entire wall is exactly what you want for your room. The result is always a stunning entire room transformation. But remember that covering part of a wall can be visually effective as well. (Think about it: that is why there is a panoramic wall mural industry!)
Another important issue that was nicely addressed with this discussion was cost. As a consumer, when you trim a few feet off your custom mural, it is money well saved.
You can be very creative and artistic with the placement of your custom photo mural. Everyone has their own unique decorating style and it should be reflected in one's living space. There are no hard and fast rules on wall mural placement, as far as I'm concerned. If there are, go ahead and break them and do what your heart tells you:)! Where you choose to hang your photo mural is your own creative decision in the end.