Building a chain driven Plasma table.

150*3.1415=471.225mm of surface times material thickness times 7850kg/m3
That gives you the weight, and at 6 meter length, even 1mm thick walls would be 22KG!
At 4mm, that is 88Kg, that have to be turned plus the friction and the chuck weight.
You can make it work with 1:4 reduction, but it would have to be at miserably low acceleration.
Do you have a 3D printer? Print some pulleys, find a HTD5 belt, at least 15mm wide, use a lot of walls to enforce the mount holes on pulleys.

Minimum would probably be 1:10 for a Nema 23 or 34 motor, should be usable up to 1:30.
I would reference from the other side of the equation, 2-3 meters per minute of cut feed rate for 40A to 60A plasma, so for 150mm tube that would make 4 turns per minute roughly, easily done, but for 50mm tube that would be roughly 15 turns per minute.
The good thing with steppers in this case is more torque at lower speed, so start form 3 to 5 turns per minute and see what you can get on a high side.