I'll answer from the reality-check point of view
How plausible are naturally-evolved, biological, fixed-wing flying creatures?
I find it very difficult, to say the least.
Being unable to fold your wings is a huge disadvantage on land: they are necessarily frail and would be exposed to predators and crash against obstacles on the ground.
Landing, I think, is the most difficult task for fixed wings, although, maybe, a huge moving tail could help overcome the difficulties.
Being able to fold wings costs very little: wings are light and easy to move.
Could such a creature plausibly evolve in Earth-like conditions? how much would conditions have to change for fixed-wing creatures to become plausible?
Birds with fixed wings could kind of survive in deserts with no predators nor obstacles, but would they outcompete their flexible counterparts? Flying further is usually the result of flapping. Mountains (bare, to avoid making obstacles) could help them take off, allow ascending currents to keep in the air and make landing a little less dangerous.
It's difficult to find an evolutionary pressure that could evolve motionless wings. But they could be born mobile and become fixed at adulthood: otherwise, eggs should be long.
One possibility is that evolutionary pressure stops fighting some kind of osteoarthritis-like degenerative sickness of the joints to favor some other advantageous traits (like longer, wider wings)
If so, what method would it use for propulsion? [And other ideas]
Apart from flightless birds, like penguins or kiwis which could survive with fixed wings but are out of the scope of the question, there are birds adapted to glide and can actually survive with very little flapping.
The closest you can get to the kind of bird you're looking for with existing species is Andean Condors:
I'm so majestic that all the countries in my range have chosen me as their national bird (image: Wikipedia)
They are the biggest birds on Earth by wing area (spanning about 11 ft long). A big area helps them glide effortlessly. They are adapted to mountains, very good at finding ascending currents and taking off from cliffs. If one of them had its wings suddenly fixed, it'd be more likely to have trouble landing than taking off or flying long distances without flapping (what they actually do).
Condors are apex predators (no one trying to kill them) and live in mountains, where they happen to find fewer obstacles. Their habitat favors non-flapping flight and take-off.
Perhaps what you need is a big-tailed condor (i.e., with a long, wide tail), whose wings become fixed OR are very small at the time of hatching
I don't think a bird could realistically live without any landing. Though they could hunt (or even gather) while flying, mobile wings increase maneuverability, thus increasing the chances to hunt (which would be a strong evolutionary force for wings not to become fixed).
how large could such a creature realistically become?
Apparently, birds can grow larger. Paleontologists argue that there have been birds with estimated wingspans of about $7\,m$. And the good news is that the bigger the bird, 1) the better it is at gliding, 2) the less efficient flapping becomes, 3) the more fixed their wings are