You probably won’t be able to move any plumbing around, but you could replace fixtures and other materials with stock, off-the-shelf products like you’d find in a big-box store.
Using several tall and skinny mirrors can give a room a more vertical look, emphasizing the height of the space (especially effective when you have high ceilings). Of course, using a single large mirror opens up the room by reflecting more of the opposite walls. Ultimately the choice comes down to personal preference.
Powder room sinks are primarily a place for quickly washing hands, so the size can be very small, as seen in this Vancouver space, or much more generous, as in the following examples.
Before you do anything, think about what your ideal bathroom would be. Forget constraints of money and space; just imagine what you want and what the intended purpose will be. Who is this bathroom for? Is it a master bathroom? A powder bathroom? A kid’s bathroom? How often will it get used and what will the function be?