WSJ has a post referencing a study showing that people with a 24 inch monitor were 44% faster than people with 18 inch monitors on a particular set of tasks. They said that performance started to go down again when given 26 inch monitors.
I find that most people don’t really know how to use a big monitor--especially with Windows. I’ve watch a lot of people open an application and maximize it regardless of how big their screen is. Then they jump back and forth between screens just like they would with a smaller monitor. You have to learn how to get the most productivity out of a bigger monitor–especially if you are in the habit of working with a smaller computer screen. This is especially true when you start getting to the large monitors. If you have a larger monitor, take a look at your actual usage. Are you really taking advantage of all that space?