I purchased a furnace for my shop that has an 8" chimney outlet. The chimney in my shop is 6". Is ok to reduce the chimney size?

Not a good idea. This will decrease the draft on the furnace and may cause a fire. If that happens and the insurance company finds the chimney is not to the manufacturers specifications then they will probably disallow all of the insurance claim. It is much cheaper and safer to install a 8" chimney or to return the furnace and get one with a 6" chimney.

It may be OK on a gas unit to reduce an 8" down to a 6" on a 80,000 BTU gravity vent gas unit, never on a 8" wood burner stove. That would be one big 100,000+ BTU shop furnace if it's modern and has an 8" exhaust vent, so on principal without seeing the equipment I would say NO as a matter of safety. Gas heaters usually require a city permit so the installation can be inspected by a qualified mechanical person to check it for proper operational code and safety.

Absolutely not. The company built that with an 8" as that was what was required for proper ventilation. If you cut it down to 6" it could very well fill your shop with carbon monoxide and if not kill you make you very sick. Cutting a larger hole and installing an 8" chimney in your shop shouldn't be that big a job nor that expensive.

If it is ok should the chimney be reduced at the outlet of the furnace or near the ceiling?

you can but if it was designed to have an 8 inch draft pipe, then it may smoke a little bit if you use a reducer to make it work on the 6 inch pipe. You should also be careful not to burn it full out since your draft is reduced.

yes

Almost certainly not.