Once you've resized your image, you'll need to save it. To do this, go to File, Save As. You can now name your file, choose where to save it, and choose the format in which to save it. For this file, I've chosen a JPEG format.
Once you've closed the first screen (seen in the screenshot here), you'll be asked to choose the quality of your JPEG. Make sure you choose "Maximum" for the least compression on your image.
Finally, you can go to File, Close. Your Action is now complete, so just press the "Stop" button to cease recording. And that's your Action written!