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.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
Add image file
Upload is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.