pinion seal on a 1999 Chevy blazer 4x4 ?

1 1/4" or 32mm should do it.

Replaced the one on my sons 2001 Blazer 3 months ago.

Mark the nut & flange with a marker or chisel so you get it back to the exact same spot.

If the spot where the seal rides has a groove worn into it, use a speedy sleeve over the worn spot.

If you have no clue how to do a seal replacement don't try! There is a crush washer in the differential that sets the turning torque on the ring and pinion gears. If you don't know how to remove the nut and count the turns and get the nut back to the same exact position you're going to need a new differential in a few hundred miles!!
Watch a youtube video to show you how to do this job; If not sure you understand how to do it DON't try! You'd be better off just topping up the differential fluid than risking ruining the differential!