I suspect that, if you have a nice smooth bit of spacetime with no nasty planets, nebulae or galaxies to deform your structure the limit is going to be time - after all, even if you go with an instantaneous fabricator it's going to take several million years to build something a mere million light years across, and delivery delays are going to increase with every increase in size (should it be a solid structure, you're going to have to dissemble a faif number of galaxies just for enough matter. Civilisations are unlikely to maintain the effort this long, so it would have to be programmed machines doing the building, and there probably wouldn't be anyone left to show it off to if ever it was finished. In the case of a Dyson swarm, with the individual elements merely planetary sized and the whole enclosing a galaxy you could probably build it a lot faster, but, one suspects, not within the projected lifetime of the human race. Radio telescopes can be built of many identical bits intercommunicating, 'twould be fun building a BIG!! one, even if you might not have any descendents to look through the eyepiece (or whatever) a project which can be useful from its inception, but gets better as the centuries roll on.