2021 Freightliner Cascadia Spindle Nut socket size?

I do not know that I have a whole lot for you. At this stage of the game is that not a sealed deal? Replace the whole assembly and torque to 350 foot pounds or so? You need to post the last 6 of your serial number and maybe someone can help you out.
Telling you how to use a hammer and chisel while bumping the tire with your knees is probably out of the question. Take that spacer in the middle and through it in the trash. Some of that new stuff does not work that way.
I learned from an old man in the back of a truck stop how to take two short chains and bolt them down on the shift tower bolt holes front to back. Have a piece of pipe across the frame rails with the chains over them. Then tighten the bolts and that lifts the transmission a bit. Use channel locks to turn the pipe and roll the transmission back. Replace the clutch and roll the transmission back in. Thank outside the box.

 

If you have the conmet preset hub and can get the part number for your truck, then you can scroll down on this site for the nut size.
Conmet 10036549 Spindle Nut Kit - PreSet Plus FF Steer Hub Keyway Type

 

Get a cheap caliper and measure it. This one is $8 at Home Depot. They also sell digital ones.