This is a small 1/4inch diameter tubing that is often referred to as 6.5mm or 6.3mm OD to be exact.
This line is made from PE (polyethylene) which is not only BPA and plasticiser free but it's also NSF approved.
Suitable for pressures up to 150psi/10.3bar/1000kPa this will handle mains water pressure no problem at all.
This is also compatible with the solenoid valves (Part KL09478) that we sell and can be used with the inl ine regulators (Part KL02172)  if you want to drop your mains water pressure.
This type of line is often used on water filtering systems or reverse ozmosis systems.