Most every drop ear 90 that plumbers install are of the 1/2" variety.
A 1/2" pipe nipple is similar to a 3/4" copper size. My best bet is that you are all set to go for the shower arm.
If you need a slip spout instead of a threaded nipple, you will need to remove the nipple and insert a copper pipe with a 1/2" male adaper on the end.
If you have a Delta tub spout, that can either be a threaded or soldered connection.