The best way to change the size would be to cut it on a lathe. But if sanding is your only option. Make a clamping block by boring a 1" hole in a small chunk of 2" x 4" then split it down the middle. Then you can use both halves to clamp the post in a vise or to a bench top with a c-clamp. Cut strips of wet-dry sandpaper about an 1" wide along the long side of the sheet. Sand using a "shoe-shine" motion using soap and water in a spray bottle as lubricant. Keep turning the post in quarters as you go to keep everything even.

For polishing I would start with 220 or finer depending on the amount of scratches, then work up to 1000 before buffing. To knock .2 mm off you might want to start with 80. Just make sure you are diligent about removing the scratches between each grit as 80 will leave some nasty marks. It helps to wash the post in water between each grit so you can see that the scratches are being erased and you do not co-mingle the grits.