I've never had issues until the landing points decide a craft is landed. (iirc, even undocking while tumbling across the ground does just fine) So, whatever the length of a common landing gear is. Now, I don't know what sort of aerodynamics or CoG issues there may be resulting from docking large spacecraft for example. I'm not 100% sure what you're asking. And I know that Martin has implied that landed docking will be fine in future versions.
Thanks for the response. I'm trying to undock a drogue chute from a falling capsule, so I can deploy the mains. But, when I undock the drogue chute, it goes to the sun.
The issue is caused by contact with the ground and the triggering of the "LANDED" flag in the scenario file. Therefore the minimum altitude is determined by the position of the vessel's touchdown points.