There really is no standard size and this is truly a preference to meet your method objectives

For HPLC, on the high pressure side, 1/16th inch outer diameter OD is most commonly used.
The low pressure side of the HPLC instrument mostly uses either the 1/16th inch or 1/8th inch OD depending on where you want to put the tubing on the instrument.

Depending on your column internal diameter (ID), your flow rate you want to use, your pump and detector capability, you can select from the following ID: 0.005″, 0.007″, 0.010″ or 0.020″. The most commonly used ID is 0.007″ for modern HPLC instruments with standard 4.6mm or 3.0mm ID columns.    

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