galvanized pipe replacement

[tlt3900]

I'm sure this has probably been posted before and I apologize if there is some redundancy.

I'm getting ready to remodel my master bathroom and I'm planning on replacing the galvanized pipe with PEX. my plan was to add a small manifold near the bathroom and then run PEX from there to each of the fixtures.

When i got into the attic to look around I noticed that it looks like much of the galvanized pipe looks like it could give away at any minute. There are many rust spots with white powdery residue around the spots up and down the pipes. So, now I'm looking at replacing all the pipe in the attic with PEX.

My basic questions are:

1. where there is 3/4 inch galvanized I was planning on replacing with 3/4 pex. Or should I run 1" PEX in the home runs in the attic?

2. In the master bathroom, the galvanized pipe is 3/4 inch into the wall just before the fixture where it reduces to 1/2 inch. Is it okay to run 1/2 inch PEX from the manifold that has 1/2 connections or should the PEX be 3/4 inch as far as it can go to the stop valve or shower control valve.

3. where the pipes turn down into the walls, there are stand pipes, I assume as a hammer arrester. Should hammer arresters be put in with the PEX to replace these stand pipes?

thanks in advance for the help

[jimbo]

In terns if equivalent inside diameter, 3/4 pex is closer to 1/2 copper or galv. 1" pex = 3/4 iron or copper

[hj]

The hammer arrestors were "useless" after only a few months. You probably do not need them, 1. because they were not working in the first place, and 2. because the PEX will absorb a lot of "shock". I probably would not repipe in PEX, but if I did, I would oversize it compared to the galvanized pipe sizes.