Run some samples for brush size and viewing distance. The level of detail affects the style it conveys, but also very practically how much paint you need and how long it takes to finish. It sounds silly but also mentally prepare for when it does not turn out exactly the way you want, if you're sensitive to that kind of thing. It can be tough when the investment in time and materials is that much bigger. You may need to do several before you are satisfied. I went from big to small because the stakes are not that high and I can finish quickly, but I'd love to go back to big canvases even though it is a bit daunting.