Originally Posted by Retro Grouch

1. You will for sure have to remove links when you install your new chain. The chain manufacturer doesn't know what kind of bike you have so they sell chains that are too long for most bikes and depend on you to shorten it to fit.
2. I size chains using the big/big method: Wrap your chain around the biggest chainring and biggest rear cog bypassing the derailleur. Figure out the shortest place you can rejoin your chain. Add 2 links (one inch) of chain to that. Don't forget to account for your master link (or whatever it's called). That's the shortest chain you can safely use with your drive train. When you are in the big/big, your derailleur will be pulled nearly horizontal. That's okay. That's what it's designed to do.
3. Interestingly the length of the derailleur cage doesn't factor into this chain length calculation.