How big can a person gauge their ears before they won't go back to their normal size?

Generally, 2g for MOST people is considered the point of no return meaning that after this, your skin loses elasticity and the fistula(tube of flesh inside) becomes weaker. 2g is approx 1/4"

However, even at 10g or even 4g some people's ears will not shrink up. The largest factor in insuring that your ears can go back to a rather normal size is that you do not skip gauges, you take your time stretching and NEVER force the taper in. Forcing the taper will rip your fistula and not only cause severe pain and bleeding but will heal with scar tissue causing future stretches to be more painful and reducing chance of your ears getting to a normal gauge. Have a professional do it. Make sure you wait 6-8 weeks between all stretches and soak in sea salt twice a day in your ears are sore or tender.

Granted the fact you are doing things wisely and safe you should be able to get to 2g and still be able to have them back to normal eventually.

I went to a 4 and mine closed back up to a 14, I think 0 is probably the biggest you may want to go.