This is done by creating an action from the Action menu. You need to give the Action a name and hit the record button. Photoshop will now memorize everything you do in between pressing the record button.
Load your first image, press record, then carry out the series of commands to reduce the size of your images, such as image size/resolution. After you've finished inputing your various commands stop the recording.
Photoshop now knows that every time that action is played it will recreate everything you’ve done in the recording mode. Remember to give your action a name.