I have a roughly 2.5 by 5 meter workshop with 2.5m high ceilings in which I work on electronics (so lots of soldering), do some occasional painting, and which houses a 3D printer. I want to add some ventilation in the ceiling, but I'm unsure what kind to put in. How much air should it move, and is it worth adding multiple "openings" (for instance, one over the 3D printer and one over the soldering station) in a space of this size?
If you are doing a lot of soldering then a dedicated soldering fume extraction unit would be a good idea. You can put the end of the hose right next to the soldering operation.
For an example on YouTube of someone who does a lot of soldering using one and showing how much gunk they stop you from breathing:
Many work shops including mine have no powered vents and I paint weld and solder also. (the printer is in the house next to the computer). Is it a good idea to have some powered vents sure! What is needed would be based on your personal wants and how much you want to spend. When I worked in a clean room the solder station had an exhaust vent within 6" to remove the smoke. Outside the clean room normal ventilation for the room was all that was required.
Depends some on how much stuff you're putting in the air. My father-in-law has a spray room attached to his workshop that's comparable in size to yours. He sprays paint and finish onto things he makes and has to wear a mask when doing so. So he has a large (roughly) 3000 CFM fan mounted on one end, but he's not doing this in a hobbyist way (he builds cabinets for a living so he'll spray dozens of things at once, sometimes).
A good bathroom fan can move 50-200 CFM.
If you're a hobbyist and not spraying anything (especially wood finish), you can probably get away with a good bathroom fan, ducted to the outside (either through a dryer vent or a roof vent). Know that the higher CFM ones are typically taller than your rafters, but are typically very quiet as well. I would aim for one in the 100 CFM range, given the size of the room.