Open photoshop. Click Alt + F9. In the opened window, click on the icon in the bottom row Create New Action, click Record. Next, open any file and perform all the actions that will be repeated every file in the folder (in your case Image -> Image Size). Upon completion of pressure to Stop Playing in the window action, in the bottom row of buttons.
File -> Automate -> Batch
In the upper part of the choose your recorded action.
In Source menu, choose Folder.
In the Destination menu, choose Save and Close.