Narrowest part of middle ear is -
“When seen in coronal section, the cavity of the middle ear is biconcave, as the medial and lateral walls are closest to each other in the center.”
The distances separating them are:

Near the roof 6 mm → Epitympanum (Attic)
In the centre 2 mm → Mesotympanum

           (between promontary and umbo)

Near the floor 4 mm → Hypotympanum
Hypotympanum
Epitympanum
Attic
Mesotympanum