how large can I go on exhaust pipes on my 98 chevy.
I have to replace from the manifolds all the way back.
can I go to 3 inch without hurting preformance, will it effect
gas milage?

www.fullsizechevy.com

Large exhaust pipes do not equal large HP gains. I would not go bigger than 2.5 inch on a single exhaust pipe.
Unless your truck is either:

1. A 9 second drag car
2. A high performance machine with a very high revving engine
3. A diesel

Too large exhaust is going to kill your bottom end torque and is pointless

The First thing you need to remember about your exhaust is, Is this a street legal Truck? If it is, you need have to keep your cat. If not, then you should not go with any lagrger pipe than a 2 1/2" on a small block or a 3" on a big block. Any larger, you will spend more money and not receive any performance gains.

Unless you add Long tubes the stock exhaust manifolds are pretty small so 3'' pipe is over kill. I would get a set-of LT and dual 2.5'' pipe.