I am redoing an upstairs bathroom that has galvanized steel pipes currently running to it for water supply that run straight up from the basement. I am debating between using PEX or copper for the new supply lines. From what I have researched, if i use PEX, it will slap against the wall of the 1st floor since i cannot attach braces inside the finished wall without removing drywall. Is this true? Also, if I do use copper, what size is recommended for a bathroom: 1/2" or 3/4"? What type of braces does copper need to support it? I am planning on bracing it in the basement and at the 2nd floor where it enters.
Copper pipe should be self supporting for the height of a room as long as you clip it as close to the floor/ceiling as possible, but if you've got particularly high rooms or high pressure you still might get some banging.
There are various types of pipe clip you can get, some screw into the wall others nail, you just need a suitable anchor point.
1/2" (15mm) pipe should be adequate for most purposes, but I'd go with the size closest to the current pipes. Bath taps (in the UK at least) usually have larger diameter (3/4") to basin taps (1/2"), so you might want to use 3/4" (22mm) pipe if you've got a large bath.
If your going to go with copper I would run a 3/4" trunk line up the wall, and then drop down to 1/2" to the fixtures.
As for bracing the pipe you could use pipe clamps like this
Just make sure the clamps are copper, if you mix metals you could risk corrosion and eventual leaks. You'll want to clamp the pipe as close as you can to where it enters and exits the first floor wall, though you may still get banging in the wall where there are no fasteners.
As much as you may want to avoid it, it might be better to open up the wall on the first floor and clamp which ever type of pipe you decide to use properly. It might be a headache now, but just imagine the headache when every time somebody flushes you hear the pipes bang (especially at 3 AM in your bedroom wall).