I just found out about the Mesh Display : Length feature in the "N" panel (thanks to this seemingly-migratable question) that allows one to measure the length of a face or vertice.
But my first encounter with this tool gives me complex feelings of love and hate, see how, as I was trying to measure the aspect ratio of this TV screen, I am getting
WIDTH = 20.33 and
HEIGHT = 37.57.
These values seem awfully wrong, since my betrusted eyes tell me the height should be shorter than the width.
I have tried measuring the single vertices as well, to identical results.