As far as i know, to change the image size using an action, you would first open the actions palette, obviously, click on the new action button. This will begin recording, then just go to Image size, plug in the dimesions you want and after you change the size just hit the stop button on the actions palette. Then any image you use this action with will be resized to those dimensions.
Open a file you want to perform action on, go to the actions panel, (Windows, Actions) and click on the little arrow at the top right of the panel and select "New Action". Now give it a name and click "Record"
Now perform the tasks you will want to repeat, ie Image Size, Unsharp Mask etc. Once you have finished click on the Stop button on the actions panel.
This will now leave you with a Action, you can now either open each file and run this action on it (time consuming) or put all your files in one folder, go to File, Automate, Batch.
Now select your newly created Action and Source Folder as the folder with all your original files in. Then Under destination choose a new folder and click on OK.
Voila, it will work through all your files performing the action on each and saving a copy in the new folder.