Neither of those sizes are fitted on any ford at all.
Are your brakes standard ?, what model sierra is it ?.
Sierras range from 240mm X 13mm, 240mm X 24mm, 260mm X 24mm, 278mm X 24mm, 283 X 24mm, on the fronts, to 253mm X 11mm, 273mm X 10mm, 273mm X 20mm on the rear.
tabetha