Search is your friend. You have a lot of questions in your message. Be sure to check the box sort by relevance.
As far as seat posts, they come in dozens of sizes, and almost all of them have a size stamped on them.
If it does not have a size stamped on it, if you plan to do a bike rebuild yourself, you will need some good tools. Go to Harbor Freight and buy a digital micrometer, they cost about $10. Then you will be able to check the size on just about everything.