Tagliatelle are a long-shaped pasta typical of central-northern Italy. The name refers to the verb "cut", because that's what you need to do to obtain tagliatelle: cutting by knife the fresh pasta, stretched and rolled on itself.
On April 16, 1972, the Confraternity of the Tortellino and the Italian Kitchen Academy deposited at the Chamber of Commerce of Crafts and Agriculture in Bologna the recipe and the right measures of the real tagliatelle of Bologna. That's why you can find a golden tagliatella's sample at the Chamber of Commerce. Measurements of real cooked tagliatelle therefore are 8 mm wide (equal to 12.270 part of the Tower of the Asinelli) equivalent to 7 mm when raw. Thickness, although it wasn't accurately coded, should be between 6 and 8 tenths of a millimeter.
Moreover, remember that "Spaghetti alla bolognese" doesn't exist in Italian cuisine. What exists is tagliatelle tossed with ragù alla bolognese, whose recipe was even filed with the Chamber of Commerce of Bologna in 1982.