Now that the Space Shuttle has been retired, are there any spacecraft available that can retrieve an object from orbit and return it safely to Earth?
What would be the maximum size of such an object?
There are several issues here. Volume, and mass for return through the atmosphere.
There is of course the next question of, how do I get this object?
Currently Soyuz can return about 100-150 lbs of mass. Volume wise it is very limited, since the descent capsule is very cramped and needs room for three cosmonauts.
SpaceX has reported about 2500KG downmass. In principle, they could probably fill the capsules entire volume. But the door on the cargo Dragon is the size of the CBM port which is 50 inches in diameter. So the ultimate size is limited.
In terms of 'getting' the object to be retrieved, right now, the answer is via the space station, or via something a Dragon or Soyuz can dock/berth too. Aside from unknown capabilities of the X37-B, there is no system available to approach, grapple, and the return with an object free floating. If you want to return something from the ISS, Dragon or Soyuz might be able to help. If it is a satellite in orbit on its own, not many options at all.
The CRS-2 contract includes funding for a cargo version of Dream Chaser that will be able to dock at the ISS and return measurable amounts of cargo. But same issues of docking, grappling, bay doors, etc.
SpaceX is proposing a cargo/satellite version of Starship, expecting it to capture the vast majority of commercial orbital lift. If they can meet their price point or even close, they will be able to undercut every other launch provider.
That will require a vehicle that can disgorge a payload, and will have tons of performance, and likely could recover something if someone was willing to pay for it.
ESA has Space Rider in development. First launch is planned for 2022 on a Vega-C.
Planned payload is 800 kg.