What is the biggest turbo I can use on a stock motor?

That is a really subjective question. The average answer you are going to get is something around the size of a 20g or FP green. Either of those are capable of maxing out a stock motor with proper supporting mods.

I ran a T3/T4 turbo on a motor with 2g pistons. It really depends on how hard you want to beat on it. Sooner or later you'll have more power than the stock motor can support.

Darryl

Yep. I'm running a BR500 right now on a stock 6-bolt bottom end with 2G pistons. Hasn't blown up yet. But I've only run 25 psi. We'll see what happens at 30 psi on 110 octane when the weather breaks.

The whole motor is stock from the intake to oil pan...lol
will I have to change inj. or any of the fuel sys to upgrade to bigger turbo?

That's what I thought you might be intending...

Assuming that you have a manual 5-speed car, your stock 14b injectors and other basic componentry will marginally support a 16g turbo. Your exhaust and fuel supply will be the first things to upgrade there-after.

Natedogg can back this up.

Yeah, my car was setup with a 16g full 3" exhaust and a hacked MAS on the stock fuel system. It did pretty well around 16-17 psi. Like awd4kicks said though, exhaust, intake, fuel pump, injectors, fuel control should soon follow any turbo upgrade just to be on the safe side.