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

For HPLC, on the high pressure side, 1/16 th inch OD is most commonly used and for the low pressure side either the 1/16 th inch or 1/8 th 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.