Even though Photoshop has had this feature for a while, many seem to have no idea that it exists. I've seen many people manually resize all their images which is just very tedious and time consuming.
Resize many images at once in Photoshop
The process of resizing many images at once is actually really simple.
First you'll need to go to 'File > Scripts > Image Processor'. You'll then be presented with the window you see below.
From here you will need to choose the folder that has all the images that you want to resize. You'll need to choose where you want it to save those images. Finally you can choose the quality and size of the image.
An extra option at the bottom is the ability to apply an action to all the images too. So if you have a watermark or a filter, you can quickly apply it to all the images using this.