The problem with this question is that the technical, financial and political limitations are interlinked.
Basically, given enough money and political will, we could probably build a space station of any size we wanted, up to something resembling a small moon, just by throwing enough money and rockets at it.
Mechanical stresses are not an issue, at least not before the station becomes big enough to have a significant gravity field of its own (which, just in case it's not obvious, would be absurdly huge) — we'd just need to build the station strong enough to withstand them. If we can build skyscrapers and giant cargo ships down here on the surface, where they have to withstand the Earth's gravity in addition to any other forces, we surely could build structures of similar (or much larger) size in orbit, if we could just get the necessary materials up there.
Sure, we'd have to increase our current launch capacity by a factor of thousands or millions to complete such a station in any reasonable timeframe, but in principle, that's perfectly doable. And that's not even considering any crazy-but-technically-possible alternative launch technologies like Project Orion that we could pull out of mothballs if we really wanted.
Of course, the big assumption here is that diverting such a huge amount of resources to space launches wouldn't collapse the economy or trigger some kind of an environmental disaster, and that some opportunistic political / ideological / religious movement wouldn't exploit the resulting hardship and dissatisfaction to take over and, as a side effect, kill the space station project. To make this work, we'd need a really good and really obvious reason for everybody to commit to such a project — something like an imminent attack by genocidal space Nazis from the moon just might be enough.
Basically, the point I'm trying to make here is that the maximum size of a space station is not limited by technology, except insofar as technology partly determines what is currently affordable.