Always match size of the plastic hoses to the size of devices using the OUTSIDE diameter, it's just easier to remember.
Such as a 1/2 inch size racking cane needs 1/2" OD plastic tubing. A 3/8 inch size racking cane requires 3/8" OD plastic tubing.
At least that is how a True Brew Gold equipment starter kit labled it's contents on the outside of the box in my LHBS yesterday!
Don't go to Lowe's or HD and get 3/8" or 1/2" ID tubing, because it will let air in, losing its prime!
Pogo