First, I want to say that I like your piece. It's very peaceful.
When you have the best image you can get, I would suggest opening your image in PhotoShop, Preview, or whatever image viewer you have available. Zoom in/out so that you can see what it will look like at various sizes. Then, when you're entering prices on FAA, you can enter prices for sizes that will work well, leaving larger sizes blank. If it only looks good at 12x16 or less (for example), you would only enter prices next to the 12x16 or smaller.