was wanting to put an exhaust on my 02 alero 3.4 this weekend and was wondering what size pipe is on there stock so i can get it and get a muffler and high flow cat and put it all on at my friends shop he has a bender and what not i just have to bring the materials

I was thinking stock is less than 2".

But if going to Fab an exhaust then You can use whatever size you'd like.

2.5" from the cat back works & sounds really well.
Thiers a couple choises for mufflers, I used the FlowMaster 80 series #42583. It has 2.5" in , and two 2.5" out, one on each side for duals.

You are 100% correct with the 2" pipe on there. I had a 2.25'' pipe on there a hogh flow magnaflow cat w/ o2 sencer and a borl muffler on there. It sounds great. Just my 2 cents.