You've got a pretty good start. I don't know the most common size, but it's different anyway for different applications.
There are the flat backs, which you use an adhesive to attach, then there are the hot fix which are pre-glued and you can iron them on, or use a tool to stick them on one at a time. And there are the kind you sew on, they come in more than one style, two that I can think of off hand are the flat sew-on ones with holes in either side, and there are montees which are mounted in a setting and are sewn on as well. http://www.rhinestonesu.com/size_chart.htm
this place has a size chart, when you print it make sure you turn off anything that makes the page a different size. They even have a section that is for children that's lead compliant.
As for cost, they are most expensive from places like wal-mart and hobby lobby (which is probably pretty obvious!). You can get good deals on ebay but you have to watch out.
The bigger the rhinestone, the smaller the number in the package generally. Most smaller ones have one gross (144) pieces, but some places sell them in smaller quantities, or larger.
They come in a variety of colors, and you are right, they come in both the leaded version and acrylic. Acrylic doesnt have the same sparkle but the lead in the others puts them in a grey area while the cpsia decides what they want to do.
I think in CA with lead laws you can use up to 4 grams of rhinestones on children's clothing.