Well this already happens many times in nature, although the “wings” can both flap and articulate (because nature doesn’t like stupid designs), they can and do fly without flapping at all. So your question boils down to “can these animals still work if the wings are fixed?)
The Amazon flying spider turns its flat body into a wing and glides from tree to tree. It’s gravity propelled flight without flapping. If it were permanently shaped this way, it’s aerodynamic performance would not change. However nature would need to give it new legs, and I think that can be done.
The majority of web-weaving spiders fly with no wings at all. Charles Darwin was amazed to find spiders landing on his ship hundreds of miles from shore. It’s called “ballooning,” where they simply pay out a length of silk which is caught by the wind, combined with electrostatic forces, that pulls them along.
In this case your question is reduced to, “Can nature evolve spiders with fixed wings?” I think that is a viable evolutionary development.
Some answers feel the outstretched wings will become a nuisance on the ground. Nature disagrees. One extremely successful animal leaves its flight gear outstretched all the time, and can fly without flapping:
So really your asking if we glued a dragonfly’s wings down (but still allowed them to pitch), would it still glide?
The answer is yes, but it’s Going to have to learn to forage for food because it won’t be the apex predator we know. It can climb a tree or cliff and glide for days on thermal currents in gravity-propelled flight.
But if you prefer propelled flight, nature has also done this. The squid uses hydraulic pressure to accumulate momentum in the water, then launches itself into the air, flattening it’s tentacles, arms, and fins into rigid wings for gliding flight.
In this case your question becomes, “can nature make a squid with rigid fins?” Again, I think this is viable, but not a smart design. And if nature could evolve a very lightweight squid, it could likely glide almost indefinitely on thermal currents.
Next you asked for how large such a creature can be. Honestly this is a function of how lightweight you can make your skeletal and muscular tissues. A thin exoskeleton like the dragonfly can be scaled up fairly well, but the animal will become quite delicate as it gets larger. Hollow bones of birds may also allow pteranodon-sized gliding animals. Nearly any creature which can launch itself high enough off the surface can take advantage of thermal currents and regular winds.
For the largest possible rigid wing creature you probably want to evolve a living dirigible with fixed wings and anguilliform locomotion. In this case, your question becomes, “Can a ballon-like creature create hydrogen gas?” I think this is also reasonable. If this creature exhaled H into its own balloon, and could articulate its body, it becomes a living airship.
Sorry this isn’t as cool as jet-propelled pteranodons, but I took the word “viable” seriously. I also took the liberty of making the wings articulate so it can steer (like ailerons) , but not flap. Because we don’t even have strictly “fixed wing” aircraft.