This is a view of the small edge (with a foot added for scale).
The thinking is that it is a stone from an arch. There seems to be a suggestion of a pattern on this 'inside' surface.
Assuming it is a stone from an arch, using a rough measurement of this 'inside' surface, the pictures, a pencil and a protractor, we have come up with an estimate for the inside diameter of the arch and hence width of entrance as 2.3m.
I don't know if this would fit for known entrances
(no guarantees of accuracy given!)