Theoretically the diameter of the rotor plus some room for the tail is sufficient. If it fits, it fits.
Practically, a bit of room for manoeuvring is required. Also special attention to the generated turbulence is needed in cramped spaces.
Regulatory I don't know the exact size required for the EC135 operated as air ambulance in the Netherlands. But I do know that the Dutch police helicopters are allowed to land on an area with a diameter of 1.5 $\times$ LOA (length over all) during daytime operations. They hit a lamp mast once which resulted in this investigation (in Dutch - or English text-only translation) by the Onderzoeksraad (Dutch Safety Board). Page 5 and 6 show some of the regulations in force at the time (2008).
I don't think there's a hard and fast rule for this, but it depends on a number of factors:
- Wind/ Weather conditions requiring safety margins.
- Effect of nearby objects/structures. If they are big I think they might cause unpredictable wind effects, again requiring compensation and space.
That said, I'm no helicopter pilot, so somebody else might be able to give a more detailed answer. YouTube is a pretty decent place to find some tight landings though.