What is a "Small Area" when talking about Cluster Mines?
From Codex Space Marines, under the Cluster Mine entry in the Glossary:


  ...may booby trap a single piece of battlefield terrain...


That reads as a specific, identified piece of terrain as placed during set-up. I.e. woods, ruins etc.

The only mention of "small" is the suggestion to place a small marker to remind both players of the booby trapped terrain.

I'm not sure where you have seen the "small area" - can you provide a reference (it could be a variation between printings perhaps?)