That's a 6g, see here for more information: http://en.wikipedia.org/wiki/Piercing#Body_Jewelry_Sizing
Yes, but it's not easy, and the bigger the gauge, the more it hurts to stretch.
If you don't gauge your ears they are only slightly visible. If you gauge them then they become much more visible.
when i first got my industrial piercing it was gauged at a 16 i would recomend this simply because the piercing needle is smaller and it is a smaller wound to heal than that of a higher gauge
They aren't plug ear piercings. They're just bigger gauge piercings. You can get them at a piercing shop which are usually joined with tattoo shops. Just check in your area. The price will be a bit more than standard gauge piercings. Punches are usually more than piercings but I don't know what size you're getting so I can't say whether you're getting a punch or piercing. Some shops don't do punches but most do so just ask.
If you're asking if you can stretch a second hole, then yes. The size it can reach depends on the individual's earlobe, hole spacing and like factors.
4mm sheet metal falls 24 gauge and 25 gauge. 24 gauge sheet metal is 3.175mm (0.125") thick, and 25 gauge sheet metal is 6.35 (0.25") mm thick.
Yes
6mm is equal to a 2 gauge.
A wire with a diameter of 4mm corresponds to a wire gauge of approximately 6 AWG (American Wire Gauge). The relationship between wire diameter and gauge is not linear, so different wire diameters can correspond to the same gauge.
14 gauge is bigger, the lower the gauge size, the bigger it is
18 or 20 gauge
yes
14
18 Gauge
You should use a taper. You can probably find these at a piercing studio. It is important to first stretch it to a 16 gauge, and then to a 14 gauge about a month later to prevent tearing.
14g is normal for a male lip piercing, 16g is normal for a female lip piercing.
For a standard nostril piercing, 18 g.