Can anyone help in understanding the thermal resistance at different copper size(mm2)?
What is the PCB copper size(mm2) refers?
can someone help me?
Yes, the thicker, the better.
Most datasheets follow JEDEC's guidelines for 2oz copper for the outer plane, and 1oz copper for the inner planes. So, you should expect better thermal resistance on your board if you have it thicker. Also, even if it's the same thickness, when you add multiple thermal vias, you already improve thermal dissipation and thus better thermal resistance.