What size Fitting do I need to attach to the T&P valve for my PVC pipe
Yes 3/4" pipe will thread onto the t&p valve and it should extend to within 6" of the floor. It should terminate in a conspicuous location so any pressure or temperature dumps will be noticeable. Hardware stores sell tubes made of plastic with a threaded end specially for this purpose.
Not sure what the official name is but it's colloquially known as a drip tube.