Most of the direct exoansion evaporator coil design with 9.52mm or 12.7mm tube. However for some applications,need to use 5/8 "pipe coil. Maybe design refrigerant evaporator coil of the most critical factor is the need to keep the refrigerant flow rate high enough to keep the oil in the refrigerant.
This is why so many DX coil with 3/8 " tube. Smaller pipe diameter is better for the mixture of oil and refrigerant.
However, for longer coil length, the small diameter can lead to too much refrigerant pressure drop, therefore maybe use a larger tube.
VRcooler supply variety of material options available.Like copper, aluminum, stainless steel, cu-ni, titanium