The maximum width of a vehicle or other object (pram, pushchair, wheelbarrow…) that is physically able to pass the barrier?
For your boulder blocking a dirt path in the forest, if the path just has moss growing on either side of it and you can just walk around it, then you are not really being blocked. But presumably there are trees or dense undergrowth somewhere nearby, so the boulder is blocking something from using the path, so the distance between the trees and boulder could be measured.