Ever played Homeworld? Repair corvettes.
Now, what I expect in a repair...hm, something half the size of a corvette, moderate hull plating, some flak and turret cover, but nothing against capships is light/moderate repair capability. If it were real (outside the game engine) I would expect it to maneuver over to the damaged part of the vessel and use whatever technology (nanotechnology, cranes...) to repair that part. Then move over to another damaged section and repeat. Not only is this much more cost effective and flexible than building a huge 5km ship to actually envelop the damaged ship, but is more realistic in terms of efficiency.