I avoided outdoor wall art for years because every piece I saw at a garden center looked like it would peel, fade, or rust into a stain on the fence within one Colorado summer. What changed was seeing a metal panel at a neighbor's place that had been outside for six years and still looked sharp. Turns out the category has moved past the printed tin signs I was picturing.
The big mistake is treating a patio wall like an indoor wall. Anything with a painted finish that isn't rated for weather will chalk and streak the first time it gets rained on sideways, and anything framed behind glass is just a science experiment in condensation. It also clashes hard with a lot of hardscaping that already has its own busy texture, a stacked stone wall or a heavily grained cedar fence doesn't need another pattern fighting it, so a simple metal cutout or a single bold shape reads better than a cluttered print in those spots. Stucco and painted siding are more forgiving backgrounds and can take something with more detail. As for where it earns a place, I'd put it on a covered porch or under a deep eave before I'd put it on a wall that takes direct sun and rain with nothing over it, and I'd think about the season too, because a piece that looks right against bare winter branches can look busy once everything greens up around it in June. Smaller bungalow patios do better with one piece sized to the wall rather than a cluster, while a bigger backyard with a pergola or an outdoor kitchen can handle a grouping without it looking like a yard sale.
Sizing it is the same math as inside, just more forgiving of boldness because you're viewing it from farther back, across a patio table or from a lounge chair rather than three feet away. I measure the wall, then aim for the art to cover somewhere between a third and two thirds of that width, and I always mock it up with painter's tape outside before drilling anything, because what looks right on a laptop screen looks tiny once it's thirty feet away against a whole elevation of brick. The other mistake is mounting hardware that isn't rated for exterior use, stainless or coated screws into a proper wall anchor, not whatever came in the box for drywall.
Good outdoor pieces actually improve with age in a way indoor art never does. Metal develops a real patina, wood grays out and softens, and a piece that looked a little stark and new the first spring starts to look like it belongs to the house by the third.