When I was building a scout-heavy army, I noticed that cluster-mines could be taken by my scout-bikes to mark a "small area" as dangerous terrain. However, there is no actual measurement in how big this area is in either the Space Marine Codex and the Core Rulebook. So in light of this, Is a "small area" the size of a small-blast marker, (3 inches) or is it some other dimension?
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?)