For chain length, just count the links on the existing chain. You can find chains that are sold with specific numbers of links, or the shop can shorten one for you. I suggest putting a "missing link" in there, if it doesn't already have one, as it makes future service easier.

Do check the existing chain for "stretch." There are guides online for checking it with a ruler. The reason is, if you're trying to solve a problem such as poor shifting, it's good to know if the chain is the root cause or not before spending excess money.