When it comes to what size mower deck is best for trimming a lawn, it ultimately depends on the area you are about to cut the lawn. For instance, if you use a mower deck on a small area like 1.5 to 3 acres, a 48-inch deck would be the best size. However, if you have a larger area to trim, you may need something larger, like a 54-inch deck. Keep in mind that the size of the mower deck will also affect how well it cuts the grass, so make sure to choose wisely.