I use a variety of ledge sizes in my cage. Some are small and serve mainly as steps (just large enough for my big girl to stand on, roughly the size of my fist), while others are wider and longer for her to rest/lounge on. I also have a few ledges that fit in the corners, an enclosed little house mounted to one of the walls, and a wooden bridge sorta thing that's suspended from the top of the cage. And like Amethyst, I have a couple branch perches made from apple wood. She seems to enjoy them all for different purposes.
Some of it just depends on the size and dimensions of your cage, but I think a variety of sizes is nice.