That has nothing to do with the rear derailler though. If you switched from say a 25 tooth large rear cog to a 34 (or even a 28 or 32) you'll need to add links to the chain to be able to use the big/big (or one down from big) combo. Most chains are sized using the big/big combo plus one inch (2 links). Thus an increase in big cog size needs a corresponding increase in chain length.

However, once you add all that extra chain, you'll likely find that you've exceeded the max chain wrap capacity of your rear derailler. That's why there's medium cage length road deraillers and long cage MTB deraillers. My commuter has an XTR long cage derailler to handle the triple and MTB cassette.