Will 6Mm Stretcher Close up?

Depends on what size you've stretched up to. If it's over 2G (6mm), most likely not. If it's smaller than 6mm, then there's a chance.

What size stretcher will go back to normal?

Every person is different, and many factors, such as the elasticity of your skin and the time and method of stretching, can affect this. Most people can go between 2g (6mm) – 00g (10mm) and expect their ears to revert back to a normal piercing, after a few months of healing.

What size of gauges will close up?

There are a lot of different opinions on this topic, but the majority of professionals in the body modification industry recommend never going any larger than 2 - 0 gauge if you want your ears to totally close up where you can't see through them.