Best bet is crawl underneath and measure it with a Vernier gauge. When you buy a replacement, don't buy the bits separately, buy the fully assembled one with the bearing already fitted to the carrier. When you fit it, take it for a test drive and if it vibrates, note what speed it vibrates at. Return home, crawl underneath and loosen the 4 bolts securing the carrier to the floorpan by no more than half a turn.
Take it for a short test run up to the speed (accelerate gently!) where it vibrated and then return hoe and tighten each bolt a little at a time (1/4 turn max) then next bolt and once you've done all 4, do them again, 1/4 turn max at a time until they're all tight.
Test drive once they're tight to confirm the vibration has gone. If not, repeat the loosening, test drive, tighten etc but it's usually sorted on the first time.
