self adjusting row height

HELP says that to change the height of a row to fit the contents, double-click the boundary below the row heading.
I'm not having any success, so I must misunderstand what is required here.
I would have thought that headings are the A, B, C, D.... usually in the top row of any SS.
The boundary below that would be the line between the row of headings and row 1.
I seem unable to select a horizontal line, except in the leftmost column.

Is some guidance available ?

Thank you Teylyn
I was forced to experiment and worked out a satisfactory 'modus operandi', but would be hard put to describe it accurately. Anyway, a practical solution was found.