The real question is how many heads will be running at the same time. If all of them, you'll probably be disappointed running it in pex, especially if you use 1/2" which has a smaller ID than 1/2" copper. Once you know the max flow, check it against the valve you want to use. 1/2" valve can flow about 5gpm with 1/2" copper supplies. A 3/4" valve can probably flow over twice that. Depending on the heads, you may not need it, but I think I'd run the 3/4" up to the valve, and if you don't go to a 3/4" valve, reduce to 1/2" there.