1. What kind of goldfish do I have?
Uh, hard to say. There are a lot of pictures on this site, look through these. Also a basic discussion of varieties. Keep in mind that specimens from a pet shop, particularly young ones, may look nothing like a mature show winner, even if they are of supposedly the same variety. Also different features and colors may appear. For example, many Ryukins and Telescope Eyes will develop headgrowth like an Oranda as they get old.
2. How long do goldfish live?
I have asked several very experienced Ranchu breeders this question and surprisingly they have consistently said "seven years". I think that for heavy-bodied, fancy goldfish that are aggressively stuffed with food all their lives, this might be a typical life span. For more wild-type fish like Comets and Shubunkins, well cared for in ponds, ten to twenty years is quite possible.
The Ranchu fanciers in Japan feel that a winter dormancy period is very important to a long life. So a fish in a pond would be expected to live longer than an indoor fish that never cools down and rests for a month or two.
The sad thing is that most pet goldfish (I would guess 99.9%) probably dont survive past 6 months.
3. How big do goldfish get?
For fancy, globoid-bodied goldfish, a seven to eight inch (20cm) body length is very large. Most top out at five to six inches. Wild-type fish like Comets and Shubunkins can get larger: body lengths over one foot (30cm) are not uncommon. I saw a common goldfish caught from a lake that I swear (I didn't catch it, so I have some credibility here....) must have been close to eighteen inches in length.