I would like to tap off the water line from the water heater so that I can add an outside garden value for a water hose.
The copper pipe from water heater is 3/4 inch.
What size should I use to tap off the 3/4 to add the water hose ?
1/2" is all you need, but you can use 3/4" if you want to. It is a personal decision on your part, not a requirement.