What is the minimal landing area required for an EC135 air medical services helicopter?
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.


(source: flugzeug-bild.de)
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.