Size of well head concrete enclosure?

When they drilled our well (6") they put an open-ended concrete cylinder around the head, about 2-2' tall, with a circular concrete slab on top; then covered that with about 1' of soil. Saves digging if the well needs to be repaired. The problem now is to mark the edges because we have to backhoe a trench near it.
Would anyone with pertinent experience know what the diameter of that cylinder is likely to be? Maybe there's some standard dimension that a driller would use (?) The fellow that drilled it isn't around anymore. I don't want to dig there, and probing keeps hitting rocks.
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