I think it depends on what orbit you are talking about. I have read the discussion thread started by fragilemachinery and it all addresses orbits similar to the current ISS.

However, you could put a station at L1 or L3, which are points between the earth and the moon where gravitational equilibrium could be maintained. This means a large station could be build, rotated to maintain an artificial gravity and the location would mean that gravitational stresses from the earth and moon would not limit that size, the way they do in other orbits.

The other issue that no one seems to have addressed is that lower (closer to earth) orbits are also really full of space 'junk'. This means that whenever the placement of objects is considered, NASA also has to consider the location in relationship to thousands of other objects. There are over 19,000 pieces larger than 2 inches that were tracked as of 2009. These represent a serious hazard to any vehicle or object in orbit.

So any station below 2000k altitude would have to have robust protection built in against impacts. There are some arguments (The Kessler Syndrome, eg.) that a cascade could multiply the amount of space junk exponentially. While this is debated, it is still arguable that lower orbits are simply not as safe.

So, allowing for the use of L1 or L3, the size would be limited to (a) cost or construction (you could spend trillions and build something very large) and (b) materials (I am not an engineer, but at some point the rotation of a space station to generate artificial gravity would create stress that could pull it apart.)

Allowing for that, I think there is a better solution. Build a base on the satellite that is already there, the moon. It already has gravity (albeit only 1/6 of earth) and then, once established that base could be used as a less expensive place than earth to push out to Mars, et el.

So your space station, which could still be built at L1 or L3 would only have to be a way point, a place where earth launches could be offloaded, moon launches could pick up the goods and transport.

Having said all of that, with current (not near future) technology, the best space station we can build is the ISS. Anything a lot bigger or more robust today would be cost prohibitive and technologically difficult.