Airy disc diameter = 2.44 * lambda * f,where lambda is the wavelength and f is the f-number. If you are puzzled because you thought bigger telescopes gave better resolution - smaller diffraction patterns - you are right, but so am I. This formula is for linear diameter, not angular diameter. An 8-inch f/10 telescope has twice the scale at the image plane, as a four-inch f/10, so that same-linear-size Airy disc takes up only half as much angle on the celestial sphere.
Harrie Rutten and Martin van Venrooij, 1988. _Telescope_Optics_, Willmann-Bell.